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The largest known galaxy is just 100x more massive than Milky Way. I wonder if there is some physical mechanism (some equilibrium) which limits the size of galaxies or if it is just because of limited time ( i.e. that the homogeneous mass distribution caused by inflation phase of space expansion was not able to form bigger inhomogenities by graviational instability during the time of 13.6 bilion years )

In case of stars, there is clearly a physical process which limits the size of newly formed stars: radiation energy due to fusion of hydrogen grows faster with mass of star than gravitation force. So, if a massive star core is formed it blows off the rest of the gas in the primordial nebula, so the star cannot grow anymore.

Is there some similar process in the formation of galaxies?

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As Ben Goldacre says, "I'm afraid it's a bit more complicated than that." Phil Plait (Bad Astronomy) has occasionally written about this. Astronomers don't have a firm definition for the 'border' of a galaxy, tho' certainly objects classified as belonging should show some sort of contained orbit. But it gets worse, as they also have rough categories of "superclusters," bunches of galaxies in relatively (millions of light-years) close proximity. So it may be as much a matter of opinion as it is of astrophysics. And, of course, things might look rather different in another couple billion years.

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    $\begingroup$ Well why not choose a definition and explain why galaxies do not get bigger than some limit according to this definition. This really isn't an answer and there are physical mechanisms that limit galaxy size: tidal stripping and harassment and negative feedback from superwinds to name two - but it's no expertise of mine. Presumably the distribution of dark halo sizes plays a role too. $\endgroup$ – Rob Jeffries Mar 29 '15 at 23:27

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