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Consider say this outstanding diagram from another question,

enter image description here

To begin with galaxies have a typical density D. Intergalactic space has a much lower density d.

As we know: in galaxies (with density D) most of the stuff is dark matter. Let's say for convenience it is 10:1.

So, the ratio of dark matter in galaxies is (we'll say) 10:1.

But here's what I can not find explained anywhere:

(1) In intergalactic space. First of all simply is there any dark matter? Or, do we believe that dark matter exists, simply, only in galaxies? Not between?

(2) If there is dark matter between galaxies. Do we know or believe: what is the ratio of dark matter out there? In fact, is it just the same ratio (say, 10:1) as inside galaxies? Or, is it perhaps even more dominant, or perhaps much less dominant? What is the ratio out there?

(3) If there is dark matter everywhere. Galaxies exist in filaments and walls. Does dark matter do this? or is it evenly-spread?

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  • $\begingroup$ Dark matter can't be evenly spread because if it was, galaxies wouldn't be as massive as they are and they wouldn't lens light as much as they do or have the orbits their outer stars do. Dark matter clumps (or halos) around galaxies for sure. But as to how common it is in between galaxies, I'm not sure anyone knows. Interesting question but I think, unanswerable. $\endgroup$ – userLTK Jun 11 '16 at 5:46
  • $\begingroup$ I appreciate that info, @userLTK - it would be great to see a nice answer on this from someone! $\endgroup$ – Fattie Jun 12 '16 at 12:02
  • $\begingroup$ Both of these answers are outstanding and I thank you both. I would click the bounty to both answers (It's silly that on the SO sites you can't simply click "reward".) I went ahead and clicked the bounty to the earlier answer so it won't be wasted. These answers (unfortunately in a way!) just "raise new questions!. It is very thought-provoking and again I really appreciate the answers here, thanks. $\endgroup$ – Fattie Jun 20 '16 at 12:22
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The short answer to your question is no, there is not dark matter between galaxies, at least at any appreciable level.

There is ordinary matter between galaxies, called the intergalactic medium (IGM). The IGM is an extremely dilute, hot gas that pervades the space between galaxies. The typical density of the IGM is about one hydrogen atom per cubic meter and its temperature is somewhere around 10^6 K. These high temperatures keep the IGM in pressure equilibrium with any galaxies and prevent it from collapsing onto galaxies.

Dark matter, however, is cold, and lacks the pressure support of the gas in the IGM, so it condenses onto galaxies. [1] One interesting difference between dark matter and ordinary matter is that dark matter lacks any sort of cooling mechanism. As a result, when ordinary matter collapses onto a galaxy, it can cool and condense into a relatively compact space and later form stars and planets. The dark matter, however, remains extended because it has no way to shed its gravitational potential energy. Galaxies therefore exhibit a substantial dark matter halo, where the ratio of dark matter to ordinary matter increases substantially a few galactic radii away from the center. Within the galaxy itself, ordinary matter is dominant by at least an order of magnitude, but the density of ordinary matter falls off exponentially while the dark matter falls off only with the cube of distance, leaving dark matter dominant a few galactic radii away from the galaxy.

One interesting question is whether or not dark matter halos exist in intergalactic space without a corresponding host galaxy at the center. As far as we can tell, the answer to that seems to be no, or at least, such structures must be quite small and exceedingly rare. There are dwarf galaxies (only 10^6 solar masses or so) which have enormous dark matter halos (much out of proportion to their size, relative to the Milky Way), but nearly every halo seems to have at least some ordinary matter present, no matter how feeble.

Cosmic voids are an even more extreme example. Voids are dominated by dark energy and therefore expand more rapidly than the rest of the universe. Voids tend to push any matter (dark or otherwise) into walls that separate them from surrounding voids. In a real sense, the universe is made of voids, and everything we know lies on the walls and filaments that separate them. This paper examined dark matter halos in cosmic voids and essentially found that there weren't any. In other words, they found that dark matter halos in cosmic voids have host galaxies just as often as dark matter halos outside of cosmic voids (which is to say, almost always).

[1]: To be absolutely pedantic, I wouldn't doubt that there are a few dark matter particles whizzing about in intergalactic space. But I would be surprised if its density exceeded the (already puny) density of ordinary matter in the IGM.

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    $\begingroup$ Clusters of galaxies are full of dark matter that is not associated with galactic halos. $\endgroup$ – Rob Jeffries Jun 13 '16 at 18:36
  • $\begingroup$ Very true, although the intervening space is not considered to be part of the IGM. Rather it is considered to be part of a distinct "intracluster medium" or ICM. $\endgroup$ – J. O'Brien Antognini Jun 13 '16 at 18:50
  • $\begingroup$ Ok, I would agree with that, but your first paragraph is too strong without first noting that you are essentially defining the extent of a galaxy by the extent of its dark matter. Nevertheless I still think there are extremely large variations of the baryonic to dark matter density out there and that dark matter is found "between galaxies" in the way that most people would think about them. $\endgroup$ – Rob Jeffries Jun 13 '16 at 19:16
  • $\begingroup$ This paper: adsabs.harvard.edu/abs/2012ApJ...746...38M uses cosmological N-body simulations to argue that dark matter extends well outside the virial radii of halos ("Intergalactic space is filled with [dark] matter. Tails of galaxies extend to great distances without cutoff, whereas the luminous component of galaxies has a definite cutoff radius corresponding to the cooling radius.") $\endgroup$ – Peter Erwin Jun 13 '16 at 19:38
  • $\begingroup$ @RobJeffries just curious.. would it be your opinion that baryonic matter is mostly in the galaxies (including dwarf galaxies and globular clusters) and that "intergalactic space" has a much higher percentage of dark matter? $\endgroup$ – Jack R. Woods Jun 14 '16 at 0:20
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Let me see if I can answer at least some of this.

  1. Yes, there is dark matter between galaxies. This is demonstrated by the fact that in galaxy groups and clusters, you need more dark matter than is found in the galaxies themselves to explain what's going on: in terms of why the groups or clusters are gravitationally bound in spite of the extreme velocities of the galaxies within the groups/clusters; why the X-ray-emitting gas in clusters is so hot and high-pressure and yet still confined to the cluster; and in order to explain the gravitational lensing of background galaxies by clusters. (Also supported by observations of things like the Sunyaev-Zeldovich effect in clusters.)

    (In fact, the oldest evidence for dark matter was the extremely high velocities of galaxies within clusters as measured by Fritz Zwicky back in the 1930s. This is also one of the failure modes of Modified Newtonian Dynamics as an alternative to dark matter: even if you think you can get rid of dark matter within galaxies, you still end up needing dark matter within galaxy clusters, in addition to the galaxies themselves.)

    And, of course, cosmological models indicate that dark matter accounts for about 25% of the critical density of the universe. In order to have that much dark matter, you have to have more dark matter than what's inside individual galaxies.

  2. "If there is dark matter between galaxies. Do we know or believe: what is the ratio of dark matter out there?" I think the simplest way to answer this would be to take the dark-matter/baryonic-matter ratio for the universe as a whole, as determined by current cosmological observations and models. This gives a ratio of about 5:1.

  3. "Galaxies exist in filaments and walls. Does dark matter do this? or is it evenly-spread?" Yes, dark matter does this. The reason we see galaxies in filaments, clusters, and walls is because the dark matter has gravitationally clumped into those structures; on this scale, the galaxies are basically along for the ride, following the gravitational pull of what the dark matter is doing. The reason for galaxies existing in filaments is because of the uneven distribution of the density of both regular and dark matter throughout the universe. An overdensity leads to the matter collapsing to form (for instance) a galaxy, but the overdensity can be asymmetric. This leads to the collapse happening faster in one particular direction, causing galaxies to form in filaments (see e.g. this summary paper for an overview).

    This page has images from a large-scale cosmological simulation (the Millennium Simulation). Most of the images showing filamentary structure on different scales are showing the dark matter only.

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  • $\begingroup$ Holy crap. So, considering the "galaxy rotation speed problem" . . . you're saying there's a similar problem, on the next scale up, with groups and clusters?? Basically: members (of groups/clusters are moving too fast, ie they should escape, but don't?? Is that right? Hell's bells! $\endgroup$ – Fattie Jun 19 '16 at 1:00
  • $\begingroup$ "members (of groups/clusters are moving too fast, ie they should escape, but don't?? Is that right?" -- Yes, exactly! Clusters usually have a significant amount of extra baryonic matter in the form of hot, X-ray-emitting intergalactic gas -- more mass than is in the stars of the galaxies, in fact -- but that still isn't enough mass to keep the cluster galaxies and the intergalactic gas from flying apart. $\endgroup$ – Peter Erwin Jun 19 '16 at 11:29
  • $\begingroup$ Hi @PeterErwin. Got it, thanks so much for this. I went back and did some basic Popular Science re-reading! Indeed if I'm not mistaken, in the historical picture it was the "cluster problem" which was noticed first; later the "galaxy problem" was noticed. Holy crap! It's like my whole universe has been turned inside-out. $\endgroup$ – Fattie Jun 19 '16 at 11:49

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