Why does dark matter not attract other dark matter in space, and form a giant structure?
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3$\begingroup$ Remember, we know almost nothing about dark matter. What we call "dark matter" is really an effect we see which we have no good explanation for. It might not be matter at all. $\endgroup$– DKNguyenCommented Dec 19, 2023 at 3:44
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$\begingroup$ Related: astronomy.stackexchange.com/q/14432/16685 $\endgroup$– PM 2RingCommented Dec 19, 2023 at 4:31
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$\begingroup$ Both of your statements in your question are not correct. 1. Dark matter does attract other dark matter in space: gravitationally. 2. Dark matter does form giant structures: galactic halos. So the answer to question is that you need to update your mental models for dark matter and astronomical structures. $\endgroup$– Anthony KhodanianCommented Dec 20, 2023 at 18:11
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2 Answers
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Dark matter does attract dark matter via gravitation. Now in normal matter, which is bulk neutral, when gravity bring it into contact with other matter (at speed), the atoms collide via electromagnetic forces, releasing energy in the form of light, and it can then coalesce into bulk matter.
Dark matter, as far as we know, doesn't interact non-gravitationally with itself, so it skips the "collide" step, and ofc, that precludes the radiation step (but it's dark, anyway), so it just orbits around other dark matter.
With that, it does form a giant structure: a galactic halo blob.
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$\begingroup$ If there's no repulsion, why don't they are all crushed by gravity ? $\endgroup$ Commented Dec 19, 2023 at 5:51
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4$\begingroup$ @An_Elephant because without the repulsion there is nothing to slow it down, so it continues to orbit around, or even through the center of gravity (oscillation along a line is a degenerate case of orbit with zero minor semi-axis). $\endgroup$ Commented Dec 19, 2023 at 9:39
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JEB's answer is correct in that the dark matter structures we have observed so far are those formed only by gravitational interactions. And indeed, the main way that Standard Model matter forms small-scale structure---by colliding and emitting photons which shed energy and angular momentum---is manifestly unavailable to dark matter.
But it's worth emphasizing that dark matter may interact with itself in other ways that allow it to similarly form structures. While this was not a feature of the popular dark matter candidates in the early days, we know now that the simplest explanations of dark matter are likely not correct. And more expansive theories of dark matter quite often contain self-interactions.
For example, to cite some recent work, a dark sector may contain dark versions of atoms which could form dark compact objects like stars. It's even possible that some, though not all, of the dark matter could be contained in dark disks. Too much dissipative dynamics among dark matter, or too large a fraction of dark matter acting this way can run into constraints from, for example, observations of the bullet cluster.
Nonetheless, some sort of dark matter self-interactions, and correspondingly some dissipation, should not at all be surprising. Heck, even axions form 'miniclusters'. Let me recommend this recent review to get into the topic of dark matter self-interactions. Future observations will hopefully tell us more about the matter power spectrum on even smaller scales, and reveal to us whether and how dark matter forms structures.
