I used to be an astrophysicist so I thought I'd share a little bit about what dark matter is (although I don't know yet whether my game will have anything to do with that).

Dark matter is a kind of matter somewhere in space that is not made of atoms. It's something else that does not absorb or give off light. We don't know what it is, and we've never detected a single dark matter object in isolation, but we know a lot of its properties. This is because like all matter, dark matter exerts a gravitational pull on other objects. This is similar to how we can detect and study black holes even though they don't give off light, because we can see stars orbiting the black hole.

So what do we know about dark matter? There's a lot of it: about 85% of matter in the universe is dark matter. It clumps together in galaxies, alongside stars. But instead of forming a disk, it makes a larger, rounder shape called a halo that encompasses the disk. This article has an illustration of what it might look like if you could see the dark matter halo. Dark matter does not interact the way atoms do, which means it won't coalesce to form structures the way atoms form stars and planets, so the halo is just individual dark matter particles flying through space. If you do an image search for dark matter, you may see images with filaments of dark matter connecting bright dots, which are galaxies. These are computer simulations of, I believe, the distribution of dark matter in the early universe, before it had finished gathering into halos.

I'll add that there are some people who don't believe in dark matter. However, no alternative hypothesis actually fits all of the evidence we have for dark matter, and they all require throwing out the theory of General Relativity, which is the single most successful theory in all of science and which has been verified many, many times. The people who do reject dark matter are overwhelmingly non-astrophysicists. Dark matter is accepted by a consensus of experts in the field. It's the DM in the name of the Standard Model of Cosmology, the Lambda-CDM model.

I'll admit to having a chip on my shoulder about people who have concluded, without having read a single peer-reviewed journal article on the matter, that the scientific community must be deluding themselves. But I'll leave it at that. :)