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Basically, I have this story I'm trying to write in which there are an advanced space-faring species of precursors (very original, I know), which have been scattered throughout the cosmos as their original homeworld and most of their species was destroyed. They are almost all aware of the existence of other ones of them throughout the universe, but most are isolated from each other by vast swaths of space. They love to toy with the 'less advanced' species in the galaxy, too.

Their technology is millennia ahead of the rest of the galaxy, which makes them enigmatic to most other races. They are all biologically immortal and have incredible powers rooted in highly advanced scientific technobabble, which allow them to individually and personally manipulate space-time to teleport themselves and other objects, materialize matter and energy from higher dimensions, create highly sophisticated devices from nothing, fly and other such powers.

They are all incredibly intelligent and knowledgeable, have super-advanced senses and so on. Each one individually could take down a Starfleet, destroy a planet, or with great effort, rearrange an entire solar system. Effectively, they are on the level of Bill Cipher in his physical form in terms of powers. Akin them to the Q from Star Trek, even.

Now, this is just an example. In the actual story, I'll likely tone their powers down quite a bit if need be, but still. I have this idea that their species cannot reproduce or breed (they cannot have children), nor can they reproduce VIA cloning or just by creating a new one of them with their matter manipulation powers. Though I'm having trouble justifying why this might be the case.

It could simply be that they simply don't WANT to do so, that they have accepted the fact that they are the last of their kind, but I'm not a fan of that idea. Hence this post.

Does anyone have any potential plausible explanations as to why this race or ANY race this advanced in fact, DESPITE being so advanced cannot reproduce beyond the current generation?

Thanks in advance.

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    $\begingroup$ How "plausible" is plausible? What would be un-plausible? $\endgroup$ – user535733 Jan 19 at 0:36
  • $\begingroup$ Some space-disease which wiped out reproductive capability in the species- it could almost certainly be reversed with direct matter manipulation and genetic engineering. Or they could just make clones. A deity or eldritch horror cursing them- too silly. Or the entire race just up and deciding not to have kids. Basically, anything that could reasonably be reversed with the level of technology they have, or anything too 'Deus Ex' like- EG, a God stepping in, basically. $\endgroup$ – Mostly Infinite Jan 19 at 1:25
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    $\begingroup$ SG's Asgard would like to have a word with you about your blatant attempt at ripping them off. $\endgroup$ – Euphoric Jan 20 at 13:43
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    $\begingroup$ Early in their world, r/childfree became popular on their version of reddit. The memetic devastation was so absolute that not even their advanced technology could do anything to abate it. $\endgroup$ – John O Jan 20 at 18:29
  • $\begingroup$ Does “biologically immortal” mean naturally immortal, or did they alter their own biology to be immortal? $\endgroup$ – RBarryYoung Jan 20 at 18:38

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They can, but they have reached a mutual contract not to do so.

These are, as you say, godlike beings. Any one of them that wants to cause problems would have caused problems on a galactic scale - which means that, since problems on a galactic scale are not happening, it stands to reason that all of the survivors must have reached a mutual understanding and a state of relative social equilibrium.

A newborn, however, introduces uncertainty and chaos. There is no guarantee that a child will accept the social contract that has been in operation for the past few million years. Because of this, they have reached a mutual understanding not to produce children. Since they are all practically immortal anyway, they don't really need to worry about securing their legacy.

What's worse - anyone who is willing to breach this mutual agreement is likely to have children who do the same. This means that they risk creating an entire race of godlike beings that do not limit themselves - which will quickly overrun those who do, unless they are eliminated immediately. For creatures this long-lived, producing a child is basically equivalent to setting off a biological WMD and will be met with swift and merciless retaliation from every other member of their race.

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  • $\begingroup$ Exactly. There have been quite a few stories about angry psioic babies causing much havoc. $\endgroup$ – RonJohn Jan 19 at 20:07
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    $\begingroup$ I remember that one of those god-toddlers created the concept of blackholes. Took them a while to move them to the center of the universe. $\endgroup$ – Martijn Jan 20 at 11:56
  • $\begingroup$ Like the vampire children in twilight. $\endgroup$ – InstaPoppy Jan 20 at 15:50
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    $\begingroup$ Hey, there's a whole story arc like this about the Q in Star Trek. $\endgroup$ – Phoenix Jan 21 at 6:12
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    $\begingroup$ This could easily be why they are the 'last' members, and could be what happened to their homeworld/civilization $\endgroup$ – Andon Jan 21 at 10:39
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They can easily reproduce, but the results would not be near-godlike immortal beings

There is no more complex problem than engineering perfect biological beings. They aren't just born.

While your scattered aliens possess the fruits of their super-advanced civilization, they are not really the engineers of their kind, and the super-ai that was required for the impossibly complex and precise bio-engineering of what they are... It was destroyed in the same event that wiped out most of their kind.

Simple things like self replicating factories attuned to their specific bio-signatures? They've got toys like that, and far beyond that. But this doesn't mean that they have the level of technological sophistication to create the pinnacle of their progenitors' achievements, which is the ability to create their own kind. And your remaining few are not the sort who dedicate themselves to reestablishing that process.

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    $\begingroup$ This could be made surprisingly plausible based on real science without bioengineering technobabble. Look up edible frogs and hybridogenesis: in short, edible frogs (EFs) come from reproduction between pool frogs (PFs) and marsh frogs (MFs), and EFs themselves are fertile, but breeding EFs with each other, or with either PFs or MFs, will not necessarily produce more EFs. $\endgroup$ – Rand al'Thor Jan 19 at 11:02
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They are sterile. They were the unexpected result of two different advanced species mating, and as often happens with hybrids, were more powerful and durable than either of their parents ("hybrid vigor"). However, hybrids cannot reproduce on their own, so once either or both of their parents species went extinct, no more could be created naturally.

Even clones would need to gestate, but if hybrid females even exist, they are not suited to gestating a hybrid child like one of the parents species was—possibly the one that went extinct first because gestating a hybrid invariably killed the mother.

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They are all dudes.

Imagine now you are a dude, one of several. You have powers and abilities, all of you, and you like to compete, show off and the like. It may not be that hard to imagine.

These dudes are satisfied with the company of the other dudes, each living in the galactic equivalent of mom's basement. They engage in their pastimes and hobbies, shoot snarky comments at each other, and perhaps get together for some Dungeons and Dragons and arguing about rules. None is looking to settle down with a nice girl and crank out the pups. Nay, these dudes are interested in ever more epic and dudely adventures and feats of dudeness.

The only time they might be interested in more of their kind is when one of their number is for whatever reason no longer available and they miss him. The last time that happened and a dude named Jeff went missing, they created a likeness of him with his powers that acted just like him and called him Copy Jeff to irritate him, and continued on as they were.

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  • $\begingroup$ I think I heard that story somewhere.... $\endgroup$ – Mindwin Jan 20 at 17:16
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    $\begingroup$ I think Niven's A World Out Of Time qualifies. There are Boys and Girls, both populations immortal but with physical development frozen before puberty. $\endgroup$ – Ross Presser Jan 21 at 21:50
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They could reproduce only some bilions years ago, when their race was still in its infancy, because of different cosmological parameters

In the course of some bilions years, the universe could change some parameters, for instance the Cosmic microwave background was "warmer" than it is now. And the reproductive biology of these creatures was very sensible to these values, to the point that they cannot breed anymore (and the possibility to recreate the right set of values is still beyond their advanced technologic level).

Or in alternative, they could still breed, but in order to become god-like (which was more expensive than keeping their actual god-like status) they needed a now irreproducible cosmological state, and the awareness that their offspring wnn't be able to join their status prevents them from trying to breed.

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The television show Stargate SG-1 featured a race of aliens who had just this problem. They were called the "Asgard". Their civilization had decided a long time ago that simply cloning their current bodies was superior to reproduction (no risk of genetic issues/children breaking the norm). However, they cloned for so long that they decided they didn't need sexual organs, and they saw no downside to this. Fast-forward a couple hundred to thousand years, and they realize that their DNA for cloning is becoming corrupt. Not storage system is permanent. Google "data rot".

Here is a very brief thread from Reddit explaining the issue: https://www.reddit.com/r/AskScienceFiction/comments/cvazg9/stargate_why_cant_the_asgard_reproduce/

Comments from the above post - "[Stargate] Why can't the Asgard reproduce?" on the subreddit "r/AskScienceFiction":

Highest rated

They physically don't have the "equipment" for it anymore. When they started cloning, they gradually made changes to their physical bodies, and there was no reason to keep sex organs (they were also taller when they reproduced sexually

Second highest rated

Somewhere in the distant past, the Asgard decided that cloning their bodies was a superior option to reproduction. At the time, they foresaw no immediate downside to this, and even removed their reproductive organs from their clones, likely so they could concentrate that energy on other biological processes. It's also why the current generation is so much smaller and physically frail compared to ancient asgard specimens. All the energy went to their brains, leaving the body behind.

They either saw no immediate side effects to all this, or assumed that future generations would keep studying the clone process and figure out a way to circumvent the tiny buildup of errors that come with copying a copy of a copy. Unfortunately, they did not. And they can't even modify what little they have left to kick start biological reproduction again, because their bodies simply aren't built to handle it.

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  • $\begingroup$ Good answer, but please add your summary of the linked discussion to your answer. This way future readers of your response can determine for themselves whether they want to wade up that swamp or not. Also, there is no guarantee the said discussion or subforum or even platform will be in existence in six months or a year. $\endgroup$ – elemtilas Jan 20 at 2:09
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    $\begingroup$ @elemtilas Thank you! Will do! $\endgroup$ – Flu17 Jan 20 at 2:10
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Two suggestions:

  1. Babies are born with their powers at full strength, so single- or even dual parenting is impossible (every tantrum becomes chaos/havoc/murder). It takes a large organized team of caregivers to keep an alien kid in check until it understands not to direct its destructive urges (at least not against its own kind. Of course they never really get over it, they just learn to suppress it :-)
  2. There is a total, fixed amount of superpower distributed among the aliens and so when you have a child, you have to give some of your superpower juice to your child, and also some of it is irrevocably lost. Assuming for the sake of example it takes three parents to make a baby, they all start with strength 100 (total 300) and then after the birth you have four aliens of strength 70 (total 280) and the 20 has been lost forever. So even if you might really want a kid, you're diminishing your own powers and also destroying the net strength of the race as a whole. The aliens used to be a lot more powerful until they finally figured out that they were pissing away their superpower juice by procreating.
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There's always the issue of simple math. If your species is immortal, even the slightest positive reproduction rate is going to mean your species is going to fill up the galaxy eventually unless death by accident is a thing. The simple passage of time means that your population is just going to keep growing, and growing, and growing. Which means eventually the old-timers are going to come into conflict with the young 'uns for resources. For example, if they are Bill Cipher or Q-esque tricksters, those resources could very well be less advanced species to mess around with. It's no fun conning the younger species and pretending to be ancient, mystic, and unknowable if the planet is full of other members of your species making the primitives wise to your game and making you seem less an eldritch force of nature and more a normal fixture.

It's kind of like the argument related to Fermi's paradox that even if sapience were rare, if even one sapient species had achieved economical sublight spaceflight millions of years ago they would have easily spread across the galaxy and colonized most habitable planets by now, simply because of the sheer amount of time involved for them to do it and the fact that if their civilization was spread out enough it would be difficult for any single event to wipe them all out.

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They're all actually puppets of another being located in another dimension, but they don't know it. They can do anything they want, because the other being wills or allows it, but they can't reproduce and don't even understand why, because the other being doesn't let them - and they don't even know the other being exists or that they're being controlled. Heck, they might not even realize that they can't reproduce, or might have have never considered that reproduction could even be a thing with them.

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  • $\begingroup$ Good answer. If you can point out existing stories with this concept, please cite them. It helps authors avoid treading same ground and being accused of plagiarism. $\endgroup$ – SRM Jan 19 at 20:36
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    $\begingroup$ @SRM can't think of a story that deals with this specifically, however it reminds me a lot of player characters in games. A lot of games give you control over somebody with an established story, relations, abilities, and so on. In-universe they do what they do because they've decided it. In reality, it's somebody controlling them literally outside their universe or reality. Player characters often have some rather bizarre restrictions put in for game balance but not present for non-player characters. For example NPCs might be able to have kids... $\endgroup$ – VLAZ Jan 20 at 18:42
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They've saturated the universe

Each godlike, immortal being needs a huge section of space to frolic around in with stepping on someone else's toes. The species has reached a point where adding more members would overcrowd the universe, so they don't reproduce.

Individuals need memories of the homeworld

Cixin Liu's Remembrance of Earth's Past trilogy goes into this a bit, but the basic concept is that individuals completely isolated from their homeworld will be a fundamentally different species from those in the homeworld. Not different in a biological way, but just in how they think. This immortal species might not want to make new members now that the homeworld is gone because they would be different in some way from the current ones.

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This is more of a mystical explanation than a science fiction one, but my other suggestions were already mentioned in answers.

What if all the souls available to that race were already in use?

Something like this was used in the TV show Babylon 5, when the Mimbarri believed that humans were reborn with Mimbarri souls.

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  • $\begingroup$ See also Philip Jose Farmer's Riverworld books. Souls, aka "wathans", are generated by a machine. $\endgroup$ – Ross Presser Jan 21 at 21:53
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They can and do but are limited by "quantum psychology"

It turns out that living forever is not so simple. It makes your personality converge to one of the few possible stable personalities from a small set. Let us call it {Zeus, Poseidon, Hera, Athena, ...}. Your genes or species don't matter since you can change them at will. And we "gods" do change them when we visit other worlds.

Why does this convergence happen ? Sorry little mortal, if you were a fast learner, you would need 10000 years to start to grasp the reasons. I am willing to explain that to you, but even if you didn't take so much of your time to find a mate and raise a kid, it would be pointless.

Pauli's exclusion principle applies

There cannot be two identical gods. In the same way that electrons in the same quantum state need to have different spins.

Why ? I can answer that, little mortal, but again, it would take more that a few of your lifespans.

Cloning only happens after an accident

In fact, our Zeus is very young, only a few million years old. We had to gather to make a new one, although it took a lot of time to convince Poseidon. And it turns out that we are not even the original gods. The previous ones were wiped out in a gamma ray burst while we were still a mortal species. But we eventually reached their point and understood what limited their behavior.

So here you are little mortal, stuck with the only possible gods you can ever have, no more, no less.

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  • $\begingroup$ Two identical gods in this scenario might be impossible because if you develop/ascend enough to be "a" Zeus, you become "the" Zeus (a part of it). As the personality is set, you become indistinguishable from any other instances of Zeus. If a god does not gain power by directly controlling more bodies, it all works. $\endgroup$ – Szega Jan 21 at 22:47
  • $\begingroup$ @Szega Yes, that would work. But it would not limit the number of individuals. I would rather go with having several gods slowly becoming Zeus threatens the stability. There can only be one god of each kind for somewhat uncomprehensible reasons. $\endgroup$ – Vincent Jan 22 at 10:23
  • $\begingroup$ I may not have expressed it properly. Someone becoming Zeus does not change the fact that there is only one of him. They would just become an instance of the one Zeus. The number of bodies directly under the control of Zeus might increase, but a god might be capable of multilocation anyway. $\endgroup$ – Szega Jan 22 at 10:30
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Anyone remember the Asgards of Stargate (TV) fame?

In a nutshell ...

  • through genetic manipulation the Asgards went sterile
  • to 'reproduce' they relied on cloning
  • over time the cloning degraded their genome to the point they were stricken with an incurable ailment/disease and eventually opted to commit mass suicide
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You wouldn't understand.

I'll explain.

Given their godlike abilities, it's inconceivable that they wouldn't be able to do whatever they wanted. It follows that they choose not to reproduce. And being so much further advanced than us, their intelligence would exceed ours as ours does an ant's. It would almost be an insult to try to explain their choices by human reasoning and morals. A puny human mind could no more understand their real reasons than an ant would be able to comprehend why I put on a condom during a one-night stand, apparently giving up a chance to reproduce.

Out-of-universe, it could be fascinating to write a world where people are completely used to accepting the higher beings' actions as unknowable and unfathomable, never attempting to explain them logically.

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  • $\begingroup$ I think you just described 1984. If the Party says 2+2=5, you'd better believe it, no matter what your logic tells you. $\endgroup$ – Ross Presser Jan 21 at 21:54
  • $\begingroup$ Love this answer, it's like penguins arguing about nuclear fusion. No matter how smart they are, they can't even come close to talking anything of any relevance to nuclear fusion. It'd be useless to even try to imagine what such beings want or need or choose. $\endgroup$ – a3y3 Feb 12 at 16:20
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Perhaps they are trans-dimensional beings that spawn more that one entity per "birth". When one being is born, it is benign and intelligent, the other being is cursed and evil with a lust for power. The helpful god-race eliminated it's sinful kin thousands of years ago, and chose not to procreate further, in order to keep the universe safe.

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  • $\begingroup$ Good kernel of an answer! It is a bit short and thus liable to be flagged as low quality. Please edit to improve your reasoning: how would this work, maybe how did it arise in the first place. $\endgroup$ – elemtilas Jan 20 at 2:12
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In their infinite wisdom, they have fully grokked that having children only serves to secure their lineage. Since they are immortal, they don't need to do so and consequently fail to see a point in having children.

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    $\begingroup$ To expand on this, the immortality and sterility had the same root cause, and the few that didn’t want this have died out. Later, the abilities to overcome this developed, but besides the lack of motivation was the understanding that having immature beings with such powers was a bad idea. $\endgroup$ – WGroleau Jan 20 at 18:30
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They are too aggressive for reproduction

They are very aggressive beings, including against their own species. Think bears, where the mate is often subject to aggression after the act.

Now fast-forward after a few millenia of evolution. A few harmless paw strikes to drive off the mate become a life threatening if laser claws have become commonplace.
Over the millenia, all individuals who still want sex simply died in mating accidents, and the survivors are those that have other interests - research (toying with sentients), play (toying with sentients), power (toying with sentients). There are likely also some who are not interested in toying with sentients; these simply don't interact, so they may exist as mysterious but ultimately inactive backshadow figures in the story (but they are useful as plot hooks and dei ex machina).

This has been played out to paranoia level in a story I read as an adolescent (cited here only for inspiration since it does not match your intention): The lifeform was sedentary, even revealing your location would turn you into a target for an interplanetary strike (they would reproduce via spores or something like that). These beings were actually fearful of any lifeform and on a quest for destroing all life in the universe. They end tragic: the last two specimens finally negotiate for reproduction, in a "safe", non-revealing manner, but a foul-up makes them recognize each other's location, and they kill each other in a mutual pre-emptive strike.
(Unfortunately I don't recall author or title.)

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A few options come to mind, but the one most different from other answers is that their power prevents them from getting close enough to each other to do the deed. The power interferes with others’ power - preventing them from teleporting near their kin, or manipulating spacetime as well, or perhaps even being immortal. And even if they could bear children, the child’s power would conflict with their mother’s. And since their power has a technological basis, they can’t just start a clone and then leave the system. The clone would have the right biology, but not the tech. But since the technology is bound to the race, they can’t just leave it for others to pick up.

It could perhaps be fallout from a very old war, where another godlike race was wiped out but not before bestowing this curse on the race (and probably blowing up their homeworld).

Thus this race ends up being lonely, localized Gods. Some might be benevolent, but after millennia, most probably break down. Or perhaps the ones who survived the destruction of the homeworld were deserters or exiles. Or perhaps the entire race is just a bunch of assholes.

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This advance precursor species has long since abandoned their original organic forms in favor of superior robotic synthoid bodies. These technologically advance bodies enable them to live forever and provide them with all the capabilities mentioned in your post. Unfortunately, as a necessary sacrifice, the reproductive capabilities this species once possessed has been lost, thus limiting this species to its current population.

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There is a simple explanation which works well with a being that has achieved immortality; they're all so old that they've gone through their equivalent of the menopause.

Perhaps you could also introduce into the story that the last young were born before whatever galactic event destroyed their homeworld. Perhaps you could even add to the story that the species were so focused on individual survival during this event that they all passed breading age during this event?

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The genetic engineering which gave them biological immortality took away the ability to reproduce naturally, by necessity or design (or both).

By necessity: the hyper-active immune system that kills any cell that shows the slightest sign of diverging from its true genetics towards cancer, is also completely inimical to a sperm, ovum, or fertilized egg. So they are sterile, with respect to natural reproduction.

By design: they are (or were) far-sighted enough to realize the consequences of bringing more of themselves into a galaxy of finite size, if neither themselves nor their children ever die. It's exponential growth in a finite container. It always ends badly.

The design bit probably means that they have some sort of means to obtain permission for the creation of new individuals by natural fertilization of an egg in vitro, and incubation in an artificial womb. This requires societal permission that is rarely given. For such an individual to be born, an existing individual must have died. Which happens, rarely. They are not immortal against extreme physical damage in accidents. They may even run out of motivation for continued life and commit painless suicide rather than continue.

This may be easier to understand if they are egg-layers like birds, rather than viviparous like mammals. Especially so, if their ancestors deposited eggs in a communal hatchery, and their species completely lacked any parental bonding or parental desires even before they modified their own biology.

As for cloning: it's just not done. As in, the only offence that carries a mandatory death penalty for original and clones alike. For reasons both practical and moral.

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Its not their knowhow that matters, its how they act on it

You describe a race that "love to toy with the 'less advanced' species in the galaxy." Toy implies a level of lack of caring about the consequences. So what if they create a mutant spacefaring race bent on conquering the universe. They're powerful enough to overcome this.

But how do they decide what can be overcome by force, and what is irreplaceable? It turns out that its enormously difficult to define "reproduction" in the philosophical sense with enough precision to permit the application of science. There is a reason that modern Western medical doctors, versed in all the latest knowledge science has to offer still respect something magical about the process of conception, gestation, and birth. There is a thing that we hold sacred in the middle, never quite fully defining it, even as we get down into the biochemical processes.

This is why the scientific community was aghast at the Chinese doctor who genetically engineered human children via CRISPR was jailed and shunned by the scientific community. We see something scared there and we choose not to prod it too hard. We circle it, trying to learn about it as we go.

This species could have made the mistake of underestimating this magic once. Once might be all it took. They might have thought "oh, we've got our DNA sequenced, and can 3d print cells on a whim. Let's make ourselves perfect." And so, they did. Only they found out they didn't quite understand what "perfect" was as well as they thought they did.

Perhaps they do have the ability to reproduce in the clinical sense, bearing new organic bodies which act on their own. But something is missing. This progeny now lacks something fundamental that made their race great. Perhaps, for some reason, this new generation simply doesn't think. They might be able to calculate, at a logical level, but can't meaningfully think for themselves. This is, after all, the line we draw between a 3d printable machine like a computer and a sentient being.

Something got lost. They poked at something they thought they had control of, and lost it. They thought they could reproduce it from information, and they were wrong. Now it is lost forever. And it turned out, they needed it.

They may still toy with the lesser advanced species. They have to, in order to obscure what they did to themselves from the rest of the galaxy. But they may have new intent now. Some may be malicious tricksters who strive to maintain their superior position in the galaxy, but others might be "toying" in a way which seeks to find that spark they lost in some other species, and raise them up as though they were their own children.

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They all are siblings Just to expand it a little back into the past, how about a version for them to be a bunch of kids from another Universe, who have had a journey with their parents and something happened, they were left behind, or something.
As omnipotent they could be in this plane, there is just not that much technology and/or knowledge they have to return to their plane or to communicate with it. Also, as immortal beings, they have no interest in cloning themselves. Nor do their ethics allow an in-breed.

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They are almost all aware of the existence of other ones of them throughout the universe, but most are isolated from each other by vast swaths of space.

They are so old their physical bodies has evolved separately from one another to better suit/adapt to their different environments to the point where they're no longer genetically compatible.

Cloning cannot be accomplished as the physical body is so highly evolved it requires a highly evolved consciousness to control it, something that the living members of their species developed before their bodies had evolved to their current level. For example, their appendix which they use to control space around them requires input from the mind to prevent it interacting with space in the wrong way with potentially devastating consequences to itself, something a new born does not posess.

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"biologically immortal" means you've cured cancer, which brought a halt to genetic mutation and therefore evolution.

I'm not sure if that can be directly weaved into why the entire species is sterile, but if the only ones you'll ever have a chance to meet have been frozen aboard a space ship for the last hundreds or millions of years that it takes to get anywhere (or longer) then they're definitely sterile by now.

The original Cosmos covers this pretty well. This distances involved and the minuscule amount of time, in compassion to the age of the universe, that 'space faring civilizations' might exist for, leaves a dishearteningly small window in which chance allows for us to meet extraterrestrial civilizations. Be careful not to un-handwave it too much.

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As a natural consequence of advanced medicine and Darwinism over many generations. With infertility treatments allowing infertile couples to conceive, they pass down their infertility problems to future generations. With ubiquitous access to infertility treatments, there is no evolutionary pressure to maintain fertility and species-wide fertility continues to drop until eventually fertility issues become too severe for medical treatments to overcome.

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Radiation caused mass-infertility

Say, they live in a small enough planet / spaceship, and there is a massive accident related to something radioactive. The accident irradiates them bad enough that everyone becomes infertile. Or they have/would cause severe deformities or other issues that "prevent" reproduction.

Perhaps they were on the verge of harnessing fusion power, but something went wrong. Or they were collecting solar power in some high-efficiency experiment that caused the accumulator to explode in some unexpected event.

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Their bodies, while physical in nature, are held together by their life-force's energy (their consciousness). Each of these beings have had to ascend, as it were, from their biologically-based immortal bodies to something more advanced. While they still perhaps look the same as other biologically-based life forms, they are actually far beyond what even they started out as back in time. Over the millennia, as they have found their true link to the universe, they have found the way to truly become immortal, by releasing their consciousness from their bodies.

As for the reason for why they still retain a body, even if only as a front, it is to be able to relate in some manner with other species throughout the universe, and to not forget their origins.

As for the reason they are the last of their kind: They have (unknowingly) ran up against a wall: That of entropy. Even their life-force (consciousness) will eventually decay as entropy takes hold in the universe. Given their exceedingly long (and seemingly infinite) lifespans, they cannot reproduce because they have already passed the point in time where they had the "excess energy" to reproduce, and are slowly withering away at a rate that not even they can fathom or detect.

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The Singularity

At some point in history the race left their physical bodies for a digital existence as a means to avoid death.

Since they're functionally immortal, they don't have a need to reproduce and lost the desire for children.

They make themselves bodies as they desire and mind transfer in and then recycle or store the body then they get tired of it.

Since immortality, there is really no return on investment for children and you could be creating a potential future rival in the future.

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They've become atheists and reject the "divine spark" of new life

As you may have observed on Earth, societies that have developed the technology to alter or manipulate the reproductive process have not been wise enough to leave it alone -- widespread contraception, sterilization, and abortion are the result. But at the core of reproduction is a basic mystery: God creates the new life, allowing us to help.

So let's fast forward a few centuries. Technology has gotten so good that we can not only tweak fertility but can actually design a child's body on a computer and assemble it molecule-by-molecule in some kind of 3D printer. Atheists would attempt to do so, reasoning that it would surely be better for the child than to "roll the dice" on a random fertilization. Or just as likely, they'd decide that there were no "dice", there can't be any "divine spark" because that's religious talk and obviously can't be admitted into serious conversation. So they wouldn't allow anyone to be naturally fertile, or any child to be created in a process that wasn't 100% planned.

Basically, the result is that they gain so much control of everything in their lives, that there's no room for a miracle to occur.

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