Conventional matter to dark matter ratio, outside of galaxies

Question

Consider say this outstanding diagram from another question,

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?

Answer 1

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.

Answer 2

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.