Most galaxies are found in clusters, and we can similarly infer the presence of yet more dark matter in between the galaxies in these clusters by its effect on the motion of the galaxies. When we add up all this dark matter, we still get only about one tenth of the amount required to halt the expansion. There may, however, be yet more dark matter distributed fairly uniformly between clusters of galaxies. This dark matter could bring the density of the universe, up to the critical value needed to stop the expansion. The critical density would seem the most natural value, and it is what the inflationary model predicts.
