I'm aware that voids are relatively empty regions of the universe, but just how empty can they be? Wikipedia states that voids have 'few or no' galaxies, but I can't find much else.

To make my question specific, suppose I were in the centre of a large void, way outside of the few galaxies that may still exist in this void. Would I occasionally encounter asteroids (if I may call them such), rogue planets, or even some occasional intergalactic star?

Remark: I'm aware that this question is a bit of a one-up of What is there in the intergalactic space?. If this is too much of a duplicate, let me know and I remove the question.


The Wikipedia article on voids is pretty good (though IMO unusually awkwardly written.) The key thing is that voids are not empty, they are just large volumes which have a lower density (typically around 10% of average) compared with the rest of the universe.

These low density areas still contain stars and galaxies, just fewer of them and the galaxies they contain tend to be smaller. The Lambda-Cold-Dark-Matter (LCDM) model, which is currently favored as our best description of the universe, predicts that the voids should not have many large galaxies in them, but should have a fair scattering of small, diffuse galaxies (SDGs). (A small diffuse galaxy is, well, small compared with the Milky Way -- sized more like the Magellanic Clouds or one of the small galaxies which are being devoured by the Milky Way today such as the Sagittarius dwarf. They are also more spread out than the Magellanic Clouds.)

Here's an interesting senior thesis reporting on a search for small diffuse galaxies in voids. (They think they found some, but need confirmation.)

You ask what the night sky would look like from within a void. First, we'll assume you're in one of the SDGs, though this wouldn't make a huge difference. What you'd see mostly is whatever planets share your solar system -- they'd be comparable in brightness to our own planets from Earth. You'd also see a few dim stars. SDGs tend not to have much star formation, so there would be very few young stars and thus very few intrinsically bright stars. The visible stars would all be ones which happened to be nearby. And because SDGs are diffuse, there'd be many fewer nearby stars. At a guess there would be 20-200 naked eye stars, with only a handful of magnitude 3 or brighter. (Compare that with 6000 naked eye stars visible from Earth.)

There would be no naked eye galaxies. The SDG itself would not appear as a milky way, but more like the Zodiacal Light -- a vague glow dimly visible under exceptional conditions.

This would be a world whose astronomy would be heavily focused on the planets.

(Planetary formation might be quite different in SDGs, but I don't know even to try to speculate on how it would be different. So I assume that the planetary system would be similar to ours. YMMV.)

  • $\begingroup$ Thanks for the response. Why would you expect planetary formation to be diffent in the first place? $\endgroup$ – guest Jun 20 '19 at 14:10
  • $\begingroup$ @guest With a lower matter density, the GMCs from which stars form may tend to be smaller which may produce fewer planets. Second, the stellar evolutionary history may be skewed towards lower-mass stars which would produce a lower proportion of heavy elements. $\endgroup$ – Mark Olson Jun 20 '19 at 14:36

I can address the part of the question that states "have 'few or no' galaxies, but I can't find much else".

In recent years there has been a lot of work published on void galaxies. Identifying void galaxies is not easy: you first have to find voids and then you have to be able to establish whether a galaxy is or is not located in one of those voids. One way of doing this is to use a geometric definition of a void as described in this paper: https://www.astro.rug.nl/~weygaert/vgs_jenam_weygaert.col.pdf

(I am a member of the Groningen Group, led by Rien van de Weygaert, who are responsible for that work, but I am not a co-author). The "void galaxies" found in this way are truly isolated and not selected by virtue of brightness or surface brightness, so it's a fairly objective sample. This sample of void galaxies was studied in the following papers (more technical): https://arxiv.org/abs/1204.5185 https://arxiv.org/abs/1410.6597

Given the difficulty of finding even these galaxies, it is unlikely that we could find isolated stars or planets, even in our "Local Void" - we lie on the border of that.

This picture is a slice through a 3D reconstruction of the density field in a relatively small volume surrounding our galaxy:

enter image description here

The slice cuts through a large number of voids, some of which we can identify from ealrier discoveries. The voids are indeed almost empty of galaxies. The image is the work of Johan Hidding, a member of the Groningen Group. See the paper: https://arxiv.org/pdf/1611.01222.pdf

  • $\begingroup$ Thanks for the info, it was very fascinating. I have yet to take a look at the paper. I went with the popular vote when choosing one answer. $\endgroup$ – guest Jun 20 '19 at 14:10

Apart from what you read in Wikipedia, nobody knows. All the things you mention could be in there, but these voids are just too far away to detect them. Commonsense dictates that like the rest of space, they must be seething with neutrinos, photons & other particles. QM mechanics will also tell you that space,including the voids,is seething with virtual particles.

At the risk of being condemned for heresy, I have to admit that so far as virtual particles are concerned, I'm an atheist. I also have a suspicion that in some of these voids; the Boötes void for example, there is something mysterious in there or has passed through there to create the void, but that is speculation.

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    $\begingroup$ Virtual particles are not what you think. $\endgroup$ – StephenG Jun 8 '19 at 5:21
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    $\begingroup$ The OP never mentioned quantum theory and I can't see any relevance here. $\endgroup$ – StephenG Jun 8 '19 at 5:23
  • $\begingroup$ My point was that the voids are not full of nothingness & the total amount of particles in them probably adds up to a fair amount of mass,but virtual particles add nothing to it & are not universally accepted. $\endgroup$ – Michael Walsby Jun 8 '19 at 9:20
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    $\begingroup$ It's not true that "nobody knows". We have plenty of observational, theoretical, and numerical constraints on this. $\endgroup$ – pela Jun 10 '19 at 17:42
  • $\begingroup$ It IS true that nobody knows! if the things guest contributor talks about are there,& they might be,explain to us how anybody would know? Only things the size of galaxies are visible,so who is this somebody you claim knows what's in there?. $\endgroup$ – Michael Walsby Jun 10 '19 at 18:13

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