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This question already has an answer here:

Galaxies have supermassive black holes at their cores exerting huge gravitational field. What is the reason that galaxies don't contract towards the center? What prevents this from happening.

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marked as duplicate by stafusa, Qmechanic Oct 11 '18 at 12:47

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Angular momentum is certainly the most important thing opposing collapse, but it is not the whole story, and nothing stabilizes galaxies forever. They do contract very slowly. By the time hell freezes over, galaxies will all have collapsed.

Consider a global cluster that has little/no angular momentum. The statistical distribution with respect to radius and velocity is Maxwellian: $\propto \exp [-\beta (m\Phi -\tfrac{1}{2}m{{v}^{2}})]$ where ${{\beta }^{-1}}$ denotes the average energy per degree of freedom and $\Phi (r)$ the gravitational potential. Heavier stars are concentrated near the center and move slowly. The cluster will never reach a true equilibrium because the lightest, fastest stars can boil off and escape, taking energy with them. The main effect of angular momentum is to make a globular cluster somewhat oblate.

Galaxies tend to be flatter rather than globular, so now consider an accretion disk consisting of rocks in roughly circular orbits around a central mass. To first order, the rocks orbit at $v=\sqrt{GM/r}$, but they can exchange momentum whenever there they have a close encounter. You can model this effect with an additional velocity diffusion term, much like shear viscosity in a fluid. The extra force opposes the orbital velocity and promotes contraction of the disk. (The distribution of dark matter changes the picture a little, because it tends to flatten the galactic rotation curve, but there will be a “viscous” effect so long as $v\ne \omega r$ with $\omega $ independent of radius.)

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Conservation of angular momentum. In order to fall to a lower orbit an orbiting object must lose considerable angular momentum. Otherwise it will continue to orbit rather than falling in.

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Galaxy will not fall into galactic center because it has centripetal force equal to centrifugal force and it has conserved angular momentum as well as gravitational forces holding galaxy

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