# Is there a problem with interpreting dark matter as hidden dimensions?

Is there any solid, scientific counterargument against the hypothetical explanation that attributes dark matter as matter in hidden planes of existence?

I am trying to construct a world in which there are creatures and objects similar to ordinary objects (physically, if not biologically), and they are normally hidden in different spatial planes where only gravity can pass through, thus appearing like dark matter.

The kind of "matter" in the hidden dimension would resemble monsters and spacecrafts of a fashion not too deviant from human imagination, they also supposedly inhabit planet-like bodies, although that would not be required.

I wonder if there is any counterarguments in physics that will undermine this construction, for example, could one refute this claim by pointing out the fact that an amount of matter packed as densely as dark matter appear to be packed would surely form black-holes or other intelligible structures like stars and planet which should cause gravitational lensing effect that is different from what is observed.

• Aside from the extra pull to cater for the merry-go-round stars, everything else is just speculation. Nov 17 '16 at 2:09
• Is there "any counterarguments in physics that will undermine this construction"? There is no evidence that dark matter either resembles or looks like hidden dimensions. All physical models involving hidden dimensions don't result in dark matter. Dark matter belongs here in our dimensions, it isn't hidden away. Nov 17 '16 at 10:20

This has been proposed, believe it or not. Gravity is the weakest of the four fundamental forces, a mystery which has been dubbed the hierarchy problem. Several solutions have been floating around; one is that there are large extra dimensions through which gravity propagates. Gravity behaves according to the inverse square law in three dimensions (in the Newtonian approximation from a point source), and as $$F\propto\frac{1}{r^{n-1}}$$ in $n$ dimensions, so as $n$ grows, the force should get weaker. If gravity is the only force that can travel between dimensions, this solves the hierarchy problem. This is a case of brane cosmology, where the universe is represented as a surface embedded in higher-dimensional space, possible adjacent to other universes.

It can also be taken a step further to imply that matter from these other dimensions should interact with matter in "our" dimensions through gravity, mimicking dark matter. This fits in well with non-baryonic dark matter theories, as dark matter doesn't interact with normal matter through the electromagnetic force. Brane cosmology implies that the other three fundamental forces are "confined" to the surface of the branes. This solves the hierarchy problem by assuming that gravity "leaks", for lack of a better word, through these extra dimensions.

Fun anecdote: Models of astronomical observations over a decade ago by Qin et al. (2005) were interpreted by the authors as being evidence of three extra dimensions. Dark matter particles wouldn't directly travel through them, but would interact through the extra dimensions. The scientific community is very much not convinced. The observations still require dark matter to exist, but simply to be something that can self-interact in these extra pathways, so not quite what you're looking for, but similar.

This should not be confused with the idea of compactified extra dimensions (think tiny, rolled-up dimensions), which have also been proposed to explain dark matter. This would lead to these strange things called Kaluza-Klein states, which could be dark matter particles (see e.g. Cornell et al. (2014), although I haven't read the paper - it's largely beyond me!).

• Doesn't gravity while weak expand in a sphere predicting 3 dimensions? Jun 29 '18 at 18:46
• @Andrey That's the current understanding, yes. The Qin idea has not yet been accepted, as far as I know, or reproduced much. But it is, after all, the OP's universe, and they can change that. Jun 29 '18 at 18:54
• What i mean is that gravity is defined as m/(r^2) and surface area of a sphere is defined as 4pir^2. So gravity fills 3d spheres and dilutes over time. So you would have to change m/(r^2). At that point you would not have a universe with planets orbiting stars Jun 29 '18 at 19:32
• This is my favorite answer among every answer so far in this site. Jun 29 '18 at 19:36
• @Andrey Sure, that's how Newtonian gravity works in three dimensions. In $n$ dimensions, it's essential proportional to $r^{-(n-1)}$, as I said, and in reality, Newtonian gravity doesn't actually describe our universe in many extreme situations. Plus, in brane cosmology, it's . . . substantially more complicated. But it does say that at least what we think of as "our" universe still has three dimensions. I know I didn't explain that well, but that's the gist of it. Jun 29 '18 at 19:39

The solid scientific counter-argument has to do with your definition of hidden. The observed behavior of dark matter is that it does not or very weakly interacts with electromagnetic radiation, yet it exerts a graviational force on the rest of the universe. What mechanism do you then propose to hide normal matter so that it can interact with non-hidden universe with the force of gravity, but not the electromagnetic force?'

If the answer is magic, then thats fine. But barring magic, the only thing I can think of that fits that description is actual black holes. So if by 'hidden' you mean, hidden in black holes, I suppose that is best we could do. There are various theories that is much of dark matter is primordial black holes. But then, any 'normal matter' of monsters or spaceships inside a black hole won't be coming out any time soon.

Alternately we could suppose that there were some way a different force other than gravity could be strong enough to prevent the electromagnetic force from escaping it. A well of strong force or weak force? I don't even know how that would work, so I won't spectulate, but you are still left with the dilemma of how EM radiation cannot escape from the well of this other force yet gravity can.

All of this is pretty speculative. Keep in mind that dark matter and dark energy shouldn't be viewed as things that will actually 'exist' in future science. The concepts are nebulous and nobody really has any idea what they are; which means they sound just like luminiferous aether and the like.

Consider that modern cosmology considers dark matter and dark energy to be 27 and 68% of the universe, respectively (according to Wikipedia and this paper). It seems unlikely to me that when we finally figure out what those things are, that they are a. just one thing and b. will be referred to as simply 'dark matter' and 'dark energy'.

So, after many detours, my final answer is: if dark matter is really just normal matter hidden in a place where EM radiation cannot escape, I don't see how the 'dark matter' can escape either.

Let's just assume that "hidden" can be tuned as necessary, just for the sake of argument.

Dark matter is observed to not behave like regular matter. Regular matter (the stuff we're made of) condenses down to stars and galaxies. To do that, it interacts with itself: Gas and dust scatter of other gas and dust, eventually randomizing ("thermalizing") their motion to allow that condensation.

When we observe dark matter gravitationally, we find that it doesn't do that. It's in "halos" much larger than the normal-matter galaxies they contain. That means dark matter interacts much more slowly/rarely than regular matter.

From this, and some calculations, it appears that there are very likely no "Dark Stars" (obligatory shout-out to the incredible movie of that name) in nature. Without the ability to interact like regular matter, dark chemistry, dark biology, dark engineering would be very different. There are not likely to be any dark matter spaceships we'd recognize.