Article  Why doesn’t dark matter collapse due to gravity?

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https://bigthink.com/starts-with-a-bang/...e-gravity/

KEY POINTS: Out there in the Universe, it isn’t just normal matter that’s present, but dark matter as well: a mysterious, invisible substance that, as far as we can tell, gravitates, but doesn’t interact through any other means. When we look at the gravitational effects that massive objects have on space, we find dark matter forms a diffuse, halo-and-filament-like network of structure. Normal matter, however, collapses into stars, galaxies, planets, and much more. If dark matter gravitates, and does so the same as normal matter does, then what prevents it from collapsing?

EXCERPT: . . . In other words, even if we filled the Universe with normal matter with the electromagnetic interaction turned off, as it expanded, the matter would slow down and clump together into large, massive, diffuse structures: halos and filaments.

Does this sound familiar?

It should sound familiar; this is precisely how dark matter behaves: as normal matter with the electromagnetic interaction (and also, at least, the strong nuclear interaction) turned off! While normal matter can collapse to do things like “go splat” and “stick together” and “shed energy” and “undergo inelastic collisions,” dark matter can do none of those things. It can only attract other particles (including other dark matter particles) through gravitation, and lose kinetic energy along with the expansion of the Universe.

In other words, even though we, as astrophysicists — including me, personally, as an astrophysicist — often talk about normal matter “gravitationally collapsing,” it’s not that gravitation is the major factor in why our familiar collapsed structures formed. Gravitation is what drives the collapse, but it’s the electromagnetic (and, to a much lesser extent, the nuclear) interactions that are what permit “collapsed structures” to form. That’s the only reason why normal matter can form:
  • stars,
  • galaxies,
  • planets,
  • astrophysically real black holes,
  • stellar remnants like neutron stars and white dwarfs,
  • and cold collections of matter, including gas and dust,
in general. Without the electromagnetic interaction, normal matter would be trapped the way dark matter is: in large, diffuse, fluffy, halo-like structures, unable to collapse further... (MORE - missing details)
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