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When I was reading the Wikipedia page about dark matter, I was wondering about the nature of it and what could it be. I have a question about dark matter but first I want to list a hypothesis:

We know that dark matter makes galaxies spin faster and it is also a significant part of their mass. So has anyone observed more distant galaxies and found some odd stuff? For example, if a very close galaxy to us has 5 times more dark matter than normal matter and when we observe a distant galaxy with 3 times more dark matter than normal matter, that would mean that at earlier stages galaxies had less dark matter than today's galaxies. Or we could look at elliptical galaxies and see also some weird stuff. Has anyone observed similar things? Also, it is said that dark matter forms into clumps – what exactly are these clumps and how do they work?

Glorfindel
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Euler-Masceroni
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    Since dark matter doesn't radiate it can't clump by itself, but it can clump through interacting gravitationally with normal matter. See https://astronomy.stackexchange.com/a/55077/16685 – PM 2Ring Dec 05 '23 at 20:18
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    Check out the Bullet Cluster, it's one of our best pieces of evidence regarding how dark matter interacts. – Darth Pseudonym Dec 05 '23 at 20:23
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    @PM2Ring In fact, dark matter can clump all by itself, although to a lesser extent than baryonic matter. Pure $N$-body codes confirm this by forming structure with a striking resemblance to the observed structure hierarchy of galaxy. There are several mechanism that makes this possible (I described them here in a little detail). – pela Dec 05 '23 at 22:12
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    @pela Ah, ok. I didn't realise that such effects could operate without the DM interacting with baryonic matter. – PM 2Ring Dec 06 '23 at 05:17
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    @PM2Ring In particular "violent relaxation" (one of my favorite terms in astronomy) is quite efficient: The timescales for a pure-DM cloud to relax (i.e. reach a dynamical equilibrium) is not much longer than when including baryons (although the final configuration is much less concentrated). – pela Dec 06 '23 at 11:33

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I’m not a dark matter expert, but I think we have to clarify some fact about it.

First of all, the reason we called them dark matter is that they don’t interact with normal matters, photons, and possibly themselves. So that would lead to the consequence that they wouldn’t collapse with one another forming stars and clouds. But still, due to gravity, they are likely to form clumps. So, yeah, they do distribute inhomogeneously over large structures.

In fact, these clumps turn out to be the main structure of the universe. Cold gases are dragged by those dark matter and thereby forming galaxies and galaxy clusters.

Back to what they are. There are a large number of candidates out there, which are thought to have various properties. For example, do they really don’t interact with others, or that they might do? What is their temperature? How massive they are?

These questions all related to specific types of model, and some have already been ruled out.

For example, we suspect that the dark matter should be cooler to form the galaxy structure we observed, and therefore, CDM (Cold Dark Matter) are more preferable then HDM (Hot Dark Matter). Certain HDM candidates like neutrinos are consequently eliminated.

Another example is the MACHO, which stands for massive compact halo object. The candidates are those large objects that don’t emit that much light like stars, such as primordial black holes, brown dwarfs… etc. . These are ruled out by the observed gravitational microlensing in the Magellanic Clouds.

Currently the leading candidates are WIMPs and Axions. You may take a look at Varitasium’s video on it!

Greeddeer
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