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There have been ideas about moving galaxies, if all of the needed stars and black holes become stellar/black hole engines but due to the 1:5 ratio of matter and dark matter there will not be enough mass in the stars to take the dark matter with them, as well as the interstellar medium, unless it is collected by other means.

I was wondering if enough stars are gathered from globular clusters and other galaxies, if they as stellar engines gather around and within the galaxy being moved and the stars/black holes within the galaxy are stellar engines, if the ratio of stellar mass is higher than the galaxies dark matter and interstellar gas mass, could the whole galaxy, dark matter and all be moved?

I am imagining an arrangement similar to a very large cD galaxy, appearing as a dispersed elliptical or sphere. I understand that we don't know the exact nature of dark matter but from the observations of its interactions through gravitational bonds could this method work providing enough stellar engines are used and their close proximity to each other does not create a limit on how many stars can be used to move the dark matter?

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    $\begingroup$ Pull a cat by it's tail, the rest of the cat comes with it. $\endgroup$
    – Justin Thyme the Second
    Jan 26, 2022 at 14:33
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    $\begingroup$ Should we assume that dark matter is matter without modified gravity? $\endgroup$
    – justforplaylists
    Jan 26, 2022 at 15:42
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    $\begingroup$ @justforplaylists I think it would be easier to assume it is matter because modified gravity would involve to many speculative properties. But any thoughts on modified gravity in this situation would be interesting. $\endgroup$
    – Alan Davies
    Jan 26, 2022 at 16:28
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    $\begingroup$ Dark matter is still a hypothesis. We have some vague ideas how to interact with dark matter, but it might all be explained by confounding variables. Dark matter might not exist. That is why we can't answer in my opinion, but it'll leave great artistic freedom when writing a story/worldbuilding. $\endgroup$
    – Trioxidane
    Jan 26, 2022 at 19:19
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    $\begingroup$ With absolutely no disrespect intended, I'd suggest that this is a meaningless (or at least unanswerable) question since it depends entirely on what "dark matter" is in the universe that you are constructing. Even in the case of e.g. rocks it would be fair for your universe to have slightly different properties from the mundane one, but since there are no robust conclusions relating to what dark matter is in "our universe" simply assuming that it is the same in all possible universes is probably unsafe. $\endgroup$
    – Mark Morgan Lloyd
    Jan 27, 2022 at 10:19

3 Answers 3

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I'll argue that yes, this can be done, thanks to real-life observations of galaxies moving their own dark matter in our universe.

We know for sure that moving large amounts of baryonic matter can in turn make significant changes to the dark matter distribution of a galaxy. This has been demonstrated through a phenomenon known as "dark matter heating" - a slight misnomer, as the temperature of the dark matter isn't changing, but rather its velocity distribution. Observations of 16 dwarf galaxies (Read et al. 2019) showed that shifting large quantities of gas and dust (by way of a starburst pushing them away through radiation pressure) caused enough of a change in a galaxy's gravitational potential to shift dark matter away from the core.

This means that your plan would indeed have a chance! To actually move away a galaxy's worth of dark matter, rather than just redistributing it, would indeed require using stars from other galaxies. You could attempt to just use the gravitational pull of these stars, or you could use an analog of dark matter heating by first shifting the galaxy's gas and dust via radiation pressure and using that change to move the dark matter indirectly. In the Milky Way, the amount of gas and dust is somewhere around 10-20% the mass of stars, though; perhaps the effect would be larger in a gas-rich galaxy.

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    $\begingroup$ Ah interesting. Thanks. I was thinking about the lack of interstellar gas in globular clusters as well as dark matter being lacking, this further shows their interactions. $\endgroup$
    – Alan Davies
    Jan 26, 2022 at 18:04
  • $\begingroup$ What is temperature if not the velocity distribution? $\endgroup$
    – Daniel Darabos
    Jan 27, 2022 at 13:21
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    $\begingroup$ @DanielDarabos I should have been a bit clearer - a change in how the dark matter's moving on large scales (so bulk motion) as opposed to an overall increase in the speed of individual particles (which is maybe the more familiar notion of temperature). You could make an argument that it's totally fine terminology by way of analogy, but I wanted to make sure people weren't confused by the name. $\endgroup$
    – HDE 226868 ♦
    Jan 27, 2022 at 14:43
  • $\begingroup$ From taking a look at the article, my non-expert impression is that the center of mass of the dark matter halo does not move. They talk about the halo expanding. I don't see any suggestion that the change of particle speeds would be coordinated. I think the speed of each particle is just individually increased. It is a bit dissimilar from the temperature of normal matter, because the DM is collisionless, so particle velocities translate to (gigantic) orbit radiuses. Sorry for splitting hairs! Nice answer! $\endgroup$
    – Daniel Darabos
    Jan 28, 2022 at 15:16
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    $\begingroup$ @DanielDarabos It's not splitting hairs at all! I think you're right on both points; I do think the former (center of mass not moving) arises because the changes to the gas are isotropic, leading to an isotropic change in the halo profile; if the civilization could break that symmetry (which could be done by bringing in external stars and targeting a particular direction to shift the gas), I think a net change in the center of mass could be achieved. $\endgroup$
    – HDE 226868 ♦
    Jan 28, 2022 at 16:12
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How is Dark Matter Different than Normal Matter?

Of the 4 fundamental forces (Gravity, Electromagnetism, Strong Force, and Weak Force), Dark Matter is known for sure to exhibit Gravity, but not Electromagnetism. Whether or nor it exhibits the Strong Force and Weak Force is less certain. It is also uncertain whether is exhibits forces other than those exhibited by normal matter. The thing that makes it so mysterious is that if it behaved like other electromagnetically neutral matter, it would collapse into neutron stars or blackholes and emit some form of detectable radiation, or it would be caught up inside of stars increasing their mass and rate of fusion, but it does not seem to do any of that. There are a lot of theories as to why this is, but not enough evidence yet to support one theory over another.

To Answer Your Question

Because gravity appears to be a 2-way interaction between dark matter and normal matter, then anything that slowly accelerates a star or galaxy should drag along any dark matter with it to a degree, but most of a galley's dark matter is in the interstellar medium making the gravitational interactions between it and stars pretty weak. So, if you tried to move a galaxy, it would behave sort of the same as any stars you forgot to put an engine on. It would get dragged along a bit, but possibly not enough to keep up.

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    $\begingroup$ I was thinking of an astrophysical jet engine rather than hawking radiation for the blackholes. But if you look at galaxy mergers the dark matter joins the larger mass/ other galaxies dark matter and the stars that don't merge gravitationally form into globular clusters, this could show that all types of matter will attract to the near by larger mass if it isn't accelerating at speed in the opposite direction. $\endgroup$
    – Alan Davies
    Jan 26, 2022 at 15:12
  • $\begingroup$ @AlanDavies Dark matter does fall into galactic orbits, yes, but with stars that are under constant acceleration, you could outrun the dark matter, unless you are accelerating very slowly. $\endgroup$
    – Nosajimiki
    Jan 26, 2022 at 15:58
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    $\begingroup$ @Goodies Dark matter does attract dark matter and normal matter seemingly hitches a ride on it. $\endgroup$
    – Alan Davies
    Jan 26, 2022 at 16:30
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    $\begingroup$ @Goodies it seems to attract other dark matter, but not collapse into recognizable black holes despite not exhibiting the normal forces that prevent the formation of black holes ... why this is, is unknown. $\endgroup$
    – Nosajimiki
    Jan 26, 2022 at 17:35
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    $\begingroup$ @Nosajimiki The reason dark matter doesn't (appear to) form black holes is that it's collisionless - it can't interact via the electromagnetic force, which would allow the dissipation of energy and therefore the formation of smaller structures or black holes. $\endgroup$
    – HDE 226868 ♦
    Jan 26, 2022 at 18:17
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All Matter Attracts Other Matter.

Let's say the dark matter forms a cloud around the galaxy and the particles are only effected by gravity. Hence they are changeless and invisible. This cloud exerts a gravitational pull on all the normal matter in the galaxy that makes it harder to move. But likewise the real matter exerts a pull on the dark matter.

The pull works both ways. The net result is that if you fit a stellar engine to every large body in the galaxy and start firing, it will be more energy expensive that if the dark matter was not there. The extra energy goes into pulling the dark matter along with you.

From here it is just a matter of scale. Once all the stars break free of the dark matter cloud, how long does it take for the cloud to gravitational attract itself back to all the stars. Keep in mind you need to keep firing the thrusters over this time, to balance how the pull works both ways. Otherwise the galaxy and cloud just end up one fifth of the way between where they were at the start.

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