Conventional matter to dark matter ratio, outside of galaxies 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?
Let me see if I can answer at least some of this.


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.
"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.
"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.