We know that most of the matter in the universe has collapsed into galaxies and galaxies into groups and clusters and that a lot of matter is presently in clusters between the galaxies. But, what fraction of total matter is not represented by these discrete observed entities? Some dark matter may have velocities too high to fall into halos perhaps from tidal tails that escape mergers. There must be some relic medium in voids with a density too low to form galaxies. Some halos may be too small to form stars. Does all of this add up to a significant intergalactic medium (IGM)? Do n-body simulations give a number for this?
One might think that we could get the answer to this question by adding up all the known mass in the nearby universe and subtracting it from the CMB derived mean density of the universe times the volume used. But some recent observational studies are suggesting that the local density is well below the mean density and that this is the cause of the Hubble Constant discrepancy.
