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Earlier in history, it was said that the celestial bodies in galaxies were bound by "mutual gravity". There is still a common misconception that the gravity of supermassive black holes binds things in the gravity, similar to how planets revolve around their parent star, which of course is not true, because studies say dark matter plays an important "role" in stabilizing a galaxy at the same time supermassive black holes also plays an important role in galaxy formation.

Question: How does dark matter actually bind things in a galaxy together? What role do supermassive black holes play other than controlling the star formation in the galaxy?

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    $\begingroup$ An example: in the Milky Way, there is about $5 \times 10^{10}$ solar masses in the form of stars, about 10% as much in the form of gas, and about 10 times as much in the form of dark matter. The central supermassive black holes has a mass of only $4 \times 10^{6}$ solar masses, about one ten-thousandth of the total stellar mass and one one-hundred-thousandth of the total mass. So the SMBH has no effect on binding the galaxy together (except in the innermost few light years). $\endgroup$
    – Peter Erwin
    Feb 22, 2022 at 12:44

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In a big disk galaxy, like the Milky way, it appears that there is around ten times as much gravitating mass than can be accounted for by stars, gas, dust and everything else you can think of. This "dark matter" clearly then plays a dominant role in binding together a galaxy, because of its gravitational influence.

However, the dark matter is more spread out and extended than the normal matter. This means that the influence of dark matter is more important for binding the outer parts of galaxies than in the inner regions.

Supermassive black holes at the centres of galaxies are generally less than 1% of the total mass. As such they play almost no role at all in binding the whole galaxy. Indeed they do not even start to influence the dynamics of stars and dust (with their gravity) until one looks into the very central regions of a galaxy. If anything the power output from an accreting supermassive black hole can cause heating and act to unbind the gas near the centre.

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  • $\begingroup$ Nice ...Can you add somemore in your answer addressing another question in my edited post ? $\endgroup$
    – Kavin Ishwaran
    Feb 22, 2022 at 8:12
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    $\begingroup$ @KavinIshwaran your new question is far too broad. If you mean what role in binding the galaxy, then the answer is not much. $\endgroup$
    – ProfRob
    Feb 22, 2022 at 17:13
  • $\begingroup$ couldn't the milky way be bound together because there are some gigantic masses beyond the observable universe that are massive enough to interact with us, in some arrangement that does not result in a net acceleration. $\endgroup$
    – Hisham
    Apr 6 at 4:20

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