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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.

RogerJBarlow
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  • Related questions: https://physics.stackexchange.com/q/220058/56299, https://physics.stackexchange.com/q/15226/56299, https://physics.stackexchange.com/q/25504/56299, https://physics.stackexchange.com/q/46634/56299, and links therein. – HDE 226868 Jun 10 '19 at 00:02
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    Possible duplicate of Is dark matter around the Milky Way spread in a spiral shape (or, in a different shape)? – Bones Jun 10 '19 at 03:56
  • In all the link indicated as possible duplicate, i cannot find the answer o even a hint toward the answer to this question. – GiorgioP-DoomsdayClockIsAt-90 Jun 10 '19 at 04:46
  • @GiorgioP the accepted answer in the one Bones gives is basically identical to niels answer here... – Kyle Kanos Jun 10 '19 at 11:39
  • @KyleKanos I do not find equivalent neither the questions nor the answers. There the emphasize of the question is on "how", here on "why". There the answer is a little ambiguous as far as the "why" (compare the accepted answer and the following OmegaCentauri's comment: eveybody would remain with some doubts about what kind of interactions DM is capable of ). I remain with my opinion that there is no duplicate (although the questions touches related issues). – GiorgioP-DoomsdayClockIsAt-90 Jun 10 '19 at 17:34
  • Observational fact? Is there any observational fact for dark matter? Only evidence for dark matter is, if such a substance exists then the current theory of gravity accurately predicts galactic observations. – Lambda Jun 11 '19 at 19:12
  • I thought the evidence for DM was the rotation rate of galaxies, which is generally much larger than could be explained by the mass of the visible stars in them. I think this was noticed by Zwicky and confirmed later by Vera Rubin. (I don't know whether the mass of black holes, which account for more local elliptical orbits of stars that appear unaccompanied, was taken into account.) – Edouard Jun 12 '19 at 17:27

2 Answers2

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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.

niels nielsen
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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.

user234287
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