Could dark matter form structures the way matter does?
No, because it does not interact with itself or normal matter so it cannot "clump" the way matter does.
Are you sure ? I thought dark matter behaved the same way as normal matter with respect to gravity, and that in fact clumps of dark matter were assumed to form a good part of galaxies' mass ? or perhaps you meant "dark energy" here.
He's sure. (Or should be) Dark matter does not clump. Yes, it interacts gravitationally, but since it has no interactions that allow it to slow down, if two pieces are above escape velocity, there's no mechanism for them to capture each other.
Ah OK I suppose I was misreading the word "clump" (or "structures" in the original post) - took it to refer to any local concentration (such as dark matter haloes around galaxies), not just clumps like planets or stars.
Do we know for a fact that it doesn't clump in that restricted sense though ? MACHOs were at a time supposed to be viable form of dark matter ?
Also, not that it makes them clump, but my understanding is that "WIMPs" by definition are subject to the weak interaction in addition to gravity.
No, I meant exactly what I said.
OK. I must say I am quite confused now, but you're far more of an expert than I am so I'll just retreat from this conversation so as not to confuse OP as well
Your confusion probably stems from the facts that first, you are right that DM makes up most of the mass of all galaxies and second, you equate clumping with gravity whereas it also requires electromagnetic forces of the same kind that keep you from sinking into the chair you are sitting in and DM does not have that kind of interaction.
Yes I clarified that in my post above about clumping vs structures. And I didn't know that baryonic dark matter was now excluded as a component of dark matter.
Edit it seems it might not be according to https://en.wikipedia.org/wiki/Baryonic_dark_matter, but maybe that article is misleading
The main reason why dark matter doesn't form dense structures is that dark matter experiences little to no friction. Basically, for a particle of dark matter to move into a lower orbit, it needs to lose energy somehow. It needs to experience friction.
Normal matter does this because it interacts strongly with the electromagnetic force: in a large cloud of gas, the cloud has some temperature. That temperature causes radiation to leave the cloud, which results in a loss of energy. The net effect of this loss of energy is for the cloud to collapse inward. This causes more collisions between the molecules, which raises the temperature, which makes it lose energy faster.
With dark matter, you can't get this effect because dark matter can't radiate photons.
I said I would retreat from this thread but I feel compelled to add this, in an attempt to clarify what I believe are only apparent contradictions between the responses above. - the experts here will correct me if I'm wrong.
The answer to your question depends on what you mean by "dark matter" and by "structures".
First, dark matter (i.e. perhaps simplistically "matter we don't see")
It is currently believed to be composed of
a) a small proportion of ordinary ("baryonic") matter, such as lone planets or such. This of course behaves in the same way as ordinary matter, since it is ordinary matter. Again, this is a minor fraction of all dark matter.
b) mainly, of "cold dark matter" composed of "something" that does not interact electromagnetically and thus as explained by others, is incapable of clumping, i.e. it doesn't form structures like planets or stars at all.
c) possibly of some amount of other forms, but these would behave in the same way as cold dark matter as far as this discussion is concerned.
Second, "structures" :
a) As far as tightly bound structures (planets ans the like) are concerned, the answer is no, for the reasons explained by others, except for the small amount of baryonic dark matter.
b) Regarding large scale structures that are only gravitationally bound (galaxies and the like), then yes, very much so. Dark matter is believed to form haloes surrounding galaxies, with vast expanses of more or less empty space between them.
This is not really adding anything to what was previously said, and you probably figured it out already anyway, but I thought it might be best to clarify something that might appear confusing in the thread (it was confusing to me at least).
Dark matter coalesces, as opposed to clumps. These clouds of dark matter are believed to have formed filament that anchored galaxies creating what is known as the cosmic web.
Separate names with a comma.