A major reason for believing in the existence of dark matter is that the stars in the outer regions of galaxies are moving too fast to remain bound to their galaxies, given only the estimated mass of the luminous matter.

But what if many of these stars are not actually bound, and really are in the process of escaping the galaxies?

The loss of stars in the outer portions of galaxies at some stage in their evolution could also be a possible explanation for the "Cosmic Downsizing" mentioned in another recent question.

Given the extremely long periods of rotation, and the comparatively short length of time that we have been observing them, it must be very difficult to determine any rates of change of galactic rotations, so how sure can we be that they have reached any sort of equilibrium?

  • $\begingroup$ I don't think galaxies would be stable over long periods of time if what you are positing is true. We see galaxies at all ranges of distance and it would be really strange if they all looked similar but were flying apart. $\endgroup$
    – zeta-band
    May 22, 2018 at 23:48
  • $\begingroup$ According to references on cosmic downsizing (astro.wisc.edu/our-science/research-areas/…), older galaxies were much larger than younger ones. $\endgroup$
    – D. Halsey
    May 22, 2018 at 23:56
  • $\begingroup$ If there is a mechanism causing galaxies to fly apart, how do you suppose they formed in the first place? $\endgroup$
    – Asher
    May 23, 2018 at 2:15
  • $\begingroup$ As far as I know, galactic formation isn't completely understood yet, and it would be out of place to say much about my personal ideas, but I can easily envision that gravitational collapse could be a chaotic process, involving several unexpected phases, before finally settling down to a stable state. $\endgroup$
    – D. Halsey
    May 23, 2018 at 2:52

2 Answers 2


Astronomers have methods to determine the velocities with which stars and clouds of gas are orbiting the center of a given galaxy, so long as the galaxy presents itself to our telescopes in an orientation which supports those measurements.

Those measurements are sensitive enough to detect any outward (radial) motions as well as orbital (circumferential) motions. The findings are that the outward motions, if present, are small in comparison to the orbital velocities- that is, that the stars in the outer regions of galaxies are not in the process of escaping them.

If one were to posit nonetheless that those stars were in fact in the process of escaping, given the lifetime of a galaxy one would need to furnish some explanation of why at this point in time the galaxy has not already entirely "evaporated" and/or why the evaporation process is kicking in right now.

  • $\begingroup$ How many revolutions would it take to "evaporate" a typical galaxy ~halfway. With rotational periods on the order of a billion years, most galaxies will have only gone around a handful of times. $\endgroup$
    – D. Halsey
    May 23, 2018 at 2:13
  • 1
    $\begingroup$ rotational period of our galaxy is ~ 250k years. $\endgroup$ May 23, 2018 at 6:03
  • $\begingroup$ Here's where I got the billion-year figure: blogs.discovermagazine.com/d-brief/2018/03/14/… $\endgroup$
    – D. Halsey
    May 23, 2018 at 13:25
  • $\begingroup$ "rotational period of our galaxy is ~ 250k years" I believe that should be 250 million years, not thousands. $\endgroup$
    – D. Halsey
    May 23, 2018 at 13:29
  • $\begingroup$ my mistake, I meant M and typed K $\endgroup$ May 23, 2018 at 17:17

We observe galaxies at many different distances, and hence ages. While some are rather frayed for various reasons, it is clear that nearby galaxies are not falling apart compared to remote galaxies.

If stars were actually unbound they would move on the order of a galaxy diameter per rotation period of the galaxy. That would make a Milky Way-like galaxy visibly dissolved in just 250 million years, yet we observe nearby (and our own!) galaxy remaining plus galaxies in the same state billions of years ago. Hence this is not a major process.

  • $\begingroup$ I think my question might have given the wrong impression. I'm not asking about entire galaxies flying apart, just the stars in the outer regions where the observed velocities seem too high. $\endgroup$
    – D. Halsey
    May 23, 2018 at 13:17
  • $\begingroup$ " plus galaxies in the same state billions of years ago" Here's a report on observations that seem to contradict that: nasa.gov/topics/universe/features/galaxy-evol.html $\endgroup$
    – D. Halsey
    May 23, 2018 at 13:19
  • $\begingroup$ @D.Halsey - I was not saying galaxies remain exactly the same, only unchanged in respect to the flying apart scenario. That galaxies have changed somewhat over the past billion years is not surprising. $\endgroup$ May 23, 2018 at 19:26

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