The dynamics of a galaxy are driven by gravity. And dark matter experiences the same gravitational forces as normal matter. The effects of other forces are very small in comparison. So why is the distribution of dark and normal matter so different?

I am not asking whether the DM distribution is different - that's taken as an observed fact. I'm asking how it could have got that way.


In the case of conventional matter, particles, atoms and molecules of it can interact electromagnetically and share energy, and they can do so in response to the effects of gravity. For example, in a (gravity-driven) accretion disc, the infalling matter particles interact strongly amongst themselves and get hot; then that heat gets radiated away and the radiating particles then cool and fall further into the gravity well. In other words, they can shed energy and then gravitationally collapse.

Dark matter does not interact electromagnetically, and thus is denied this mechanism of dissipative collapse.

This difference means that the luminous matter in a galaxy can be expected to distribute itself differently than the dark matter in that same galaxy.

  • $\begingroup$ Would a discussion of the entropy involved also be illuminating? $\endgroup$ – Yakk Jun 10 at 17:43
  • $\begingroup$ Possibly, but I'm not the guy to provide it. $\endgroup$ – niels nielsen Jun 11 at 0:18

Three scenarios come to mind that could cause this. The first is an unstable or inconsistent gravitational pull from dark matter that could act as a kind of slingshot to the matter it pulled on. The second would be that the dark matter was dragged/blown away to some new location perhaps by neutrinos. The third would be a combination of the two. Think of DM behaving like other quantum particles and that its formation and destruction is a consequence of the fabric of reality. To be clear i make no claim to the validity of these guesses.

I believe the most commonly accepted cause is that visible matter tugs back and this causes dark matter to get thrown around as much as the stuff we can see.


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