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If gravitational lensing implies that Dark Matter exerts gravity onto light then why are the stars and regular matter not clustering around Dark Matter?

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  • $\begingroup$ They are clustering around DM $\endgroup$ Jun 7 '18 at 15:43
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Stars, gas and dust do cluster within dark matter: most galaxies appear to be embedded within concentrations of dark matter.

However, it's very difficult for dark matter to condense into dense concentrations since it primarily interacts gravitationally and is almost unaffected by other forces. As dark matter tries to collapse its gravitational potential energy gets converted into kinetic energy, and unlike normal matter it can't easily shed that KE through collisions which convert the KE into heat that can be radiated away. So DM tends to form large diffuse structures, not condensed lumps. The evidence for this are the haloes of DM associated with most galaxies.

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Stars and other objects do cluster around dark matter. ~90% of the mass of a galaxy is in the form of dark matter. For the purpose of visualising, you may think of dark matter as a fluid with zero drag that the stars and regular matter are immersed in and move around through.

On small scales, comparable to the size solar system, the effect of dark matter is negligible because the dark matter within the solar system is spread very thin, and there is not much total dark matter within the solar system. But when you consider the contribution of dark matter spread out over the entire galaxy to the orbits of stars around the galaxy, dark matter has a major gravitational effect on those stars' orbits.

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