Smaller fragments of the dense pancake can then collapse successively, to become galaxy clusters and individual galaxies. Where sheets intersect, large clusters of galaxies would form. This picture is called the top-down model, since the largest structures in the Universe would be made first. But if WIMPs accounted for most of the matter, smaller objects could collapse and become dense at an earlier stage. To see why, we can recalculate the minimum mass of a region that will collapse under its own gravity, by analogy with Equation 7.44.
WIMPs are unaffected by radiation pressure.
Lumps of WIMPs with the mass of galaxies or galaxy clusters could start to collapse well before comparably sized chunks of normal matter. As it escaped from the contracting clouds of WIMPs, the radiation would have taken the normal matter with it. So both of these should be quite evenly spread at recombination, and we would expect very little variation in the temperature of the cosmic background radiation. Later, as the matter became neutral, and was freed of the radiation pressure, it would fall into the already-dense clumps of WIMPs. Fluctuations in the density of normal matter could then grow much more rapidly than Equation 7.27 would allow, building up the galaxies and clusters that we see today.