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This is a follow up of Interactions within constituents of dark matter .

I wonder about dark matter, and, naturally, compare it with our observable world.

If gravitation would be the only force acting on things, would galaxies and solar systems form, and how would the look like?

From what I understand is, that a disk can only form when the things collide in some way, exchanging velocity; otherwise we would only see a chaotic bubble of things around the local center of gravity.

Sorry, I would delete this question as I could not write it in a way to make its purpose understandable, but I'm not allowed to do it.

It was to understand what dark matter would look like if there really was no internal forces other than gravity.

I would not like this question to be reopened but rather to be deleted (I am not allowed to do it, even when the closed flag falsely suggests it). I don't want to start any kind of war here.

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    $\begingroup$ We would never have existed, and you wouldn't have been able to ask that question. Without an electromagnetic force to resist gravity, everything collapses into a black hole (aka non-existence). $\endgroup$
    – David White
    Commented Feb 8 at 1:19
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    $\begingroup$ No nuclear forces means no stars. $\endgroup$
    – Jon Custer
    Commented Feb 8 at 1:53
  • $\begingroup$ @DavidWhite Or rather, stays as a singularity.... Nothing but gravity means that no expansion can occur...unless the unknown theory of quantum gravity says otherwise. $\endgroup$
    – DKNguyen
    Commented Feb 8 at 4:33
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    $\begingroup$ Here's an answer on our sister site, on the related topic of large-scale structures of dark matter: astronomy.stackexchange.com/a/14433/16685 $\endgroup$
    – PM 2Ring
    Commented Feb 8 at 5:36
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    $\begingroup$ The close reason is nonsense. The mainstream cosmological simulation community has characterized extensively what the universe would look like if gravity were the only force. $\endgroup$
    – Sten
    Commented Feb 8 at 20:24

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A universe with only gravitation would not look remotely like ours. Removing any of the other three forces would change the universe in highly significant ways:

  • Without the electromagnetic force, there'd be no atoms.
  • Without the nuclear forces, there'd be no atomic nuclei, since protons/neutrons would not stick together.

Under these circumstances there can be no stars or galaxies.

It sounds like you're asking whether dark matter can form stars and galaxies. That's a different question because dark matter's exact identity is unknown, so you can postulate interactions that make it possible. See the relevant section on Wikipedia.

If dark matter is composed of weakly-interacting particles, then an obvious question is whether it can form objects equivalent to planets, stars, or black holes. Historically, the answer has been it cannot, because of two factors:

It lacks an efficient means to lose energy ...

It lacks a diversity of interactions needed to form structures ...

However, there are theories of atomic dark matter similar to normal matter that overcome these problems.

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