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The void usually refers to the region of the Universe where the overdensity $\delta(x)$ is less than zero. What I'm curious about is whether there are virialized objects, like galaxy clusters or dark matter halos, living in such an underdense environment.

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    $\begingroup$ There are certainly galaxies in voids, so there are virialized dark-matter halos. Probably no actual clusters, though. $\endgroup$ Nov 28 '21 at 12:31
  • $\begingroup$ @PeterErwin Why galaxies can live in voids theoretically? $\endgroup$
    – Wang Yun
    Nov 28 '21 at 13:01
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    $\begingroup$ In practice any kind. Spirals seem more common in these quiescent environments. $\endgroup$ Nov 28 '21 at 21:35
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    $\begingroup$ @WangYun A void is where the average overdensity is low. There will still be local variations, though, so there can be regions within a void where the local overdensity is large enough for gravitational collapse of a halo to take place. (Such regions will be much less common in a void, of course.) $\endgroup$ Nov 30 '21 at 17:54
  • $\begingroup$ @PeterErwin Thank you for your reply. $\endgroup$
    – Wang Yun
    Dec 1 '21 at 2:12

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