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I have read this question:

To zeroth order, dark matter can only 'clump' as much as its initial energy (obtained soon after the big-bang) allows. One example of such a 'clump' is a 'Dark Matter Halo' in which galaxies are embedded. DM Halos are (effectively) always larger than the normal (baryonic) matter inside them --- because the normal matter is able to dissipate energy and collapse farther.

And this one:

The only way you can do this is to remove kinetic energy from the system. With normal matter this is done through electromagnetic interactions, which turn the kinetic energy of normal matter (protons, electrons etc.) into photons, which then escape from the system. Since these kinds of interactions do not occur for dark matter (by definition), then there is no way to get rid of kinetic energy and so the dark matter remains as a large "halo" around gravitationally clumping ordinary matter.

If dark matter only interacts with gravity, why doesn't it all clump together in a single point?

The universe didn't shrink down to a point at the Big Bang, it's just that the spacing between any two randomly selected spacetime points shrank down to zero.

Did the Big Bang happen at a point?

So at the Big Bang, we have a very odd situation where the spacing between every point in the universe is zero, but the universe is still infinite.

Now all of these agree on that dark matter cannot interact using any force except gravity. It cannot clump because it cannot lose (dissipate through these interactions) kinetic energy.

As far as I understand, the universe has no edge and no center, it is infinite. It is not like a balloon.

When you squeeze a balloon, everything in it has to go to the center because the sidewalls will push eventually everything in (even if those things wouldn't want to go closer to the center).

But this is not the way with the universe, it has no edge to do this, and no center to clump into. If dark matter cannot clump and cannot lose kinetic energy through interactions, then why would it fit into a smaller and smaller space together with ordinary matter and energy?

As the second answer says, with galaxies, dark matter remains a halo around the galaxy, because it cannot clump. This is not the case with the universe, because, there is nothing around (outside) of it. The observable universe (that we know today) used to be a very small region of space at the beginning of the universe, but it had no edge and no center, the whole universe might be infinite, and dark matter could always stay as a halo around the observable universe, if it cannot clump.

The last one says that the spacing between two spacetime points shrank down, which means clumping, but if dark matter cannot clump, how was dark matter distributed?

Just to clarify, I am only asking about the dark matter in the observable universe. If this cannot clump, then where would it be distributed when the observable universe was much smaller?

Question:

  1. If dark matter can't clump together, then where was it when the observable universe was much smaller at the beginning of the universe?
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  • $\begingroup$ Dark matter can interact with ordinary matter gravitationally. Ordinary matter can lose kinetical energy. On the long-term, the expected kinetical energy of the parts of the system is distributed evenly. So, I think, slowly also the Dark Matter can lose kinetical energy, particularly in galaxies, and it will slowly clump. $\endgroup$
    – peterh
    Jul 7 at 22:13
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    $\begingroup$ Did the Big Bang happen at a point? $\endgroup$
    – mmesser314
    Jul 7 at 22:45
  • $\begingroup$ Fermions can't collapse to a single point either. Like dark matter, they (and everything else) were created in the big bang. $\endgroup$
    – hdhondt
    Jul 7 at 23:27
  • $\begingroup$ @mmesser314 yes, thank you, exactly that is describing what I am writing "The universe doesn't have a centre: the Big Bang didn't happen at a point so there is no central point in the universe that it is expanding from. The universe isn't expanding into anything: because the universe isn't expanding like a ball of fire, there is no space outside the universe that it is expanding into." $\endgroup$ Jul 8 at 0:38
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    $\begingroup$ @shawn_halayka There is no proof for that, it is just what you think. Might read this. $\endgroup$
    – peterh
    Jul 17 at 16:19
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In the four dimensional mathematical space describing the Big Bang model there exists a singularity, where all energy/matter was in the beginning and from which, viewed in the time axis there is expansion of space.

If dark matter can't clump together, then where was it when the observable universe was much smaller at the beginning of the universe?

If the universe were only made of dark matter, because there were no other forces than gravity, it would still "clump" due to the gravitational attraction it has, but it would take forever,compared to the BB time scale. Because gravity is such a weak force with respect to the three other forces that create matter to be "clumped", clumps of matter as we know it happen as the model shows, and generate the universe we see.

The dark matter of our universe was there, in the quark gluon plasma etc, except that with the expansion, matter as we know it came into being, making "clumps" that could attract each other gravitationally because of being so massive in space.Dark matter is affected by these large masses but is hypothesized to be composed of particles that do not interact with the strong and electromagnetic interactions that tie up matter as we know it, so are just weakly tied up by gravitational attraction.

After all experiments at LHC are looking for just only weakly interacting particles ( the weak interaction is weak enough to play that role,) that could model dark matter.

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  • $\begingroup$ Thank you so much! $\endgroup$ Jul 8 at 4:57

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