Thus, we require seed density fluctuations in the early universe that allow matter to clump into galaxies and groups of galaxies. Here is a link to a set of MPEG movies showing computer simulations of large-scale structural growth in the Universe utilizing a mixture of hot and cold dark matter. The adjacent image shows the end result of such a simulation (click on the image to get a larger version).

In this simulation (Greg L. Bryan and Michael L. Norman, Grand Challenge Cosmology Consortium), filaments grow that eventually exceed 100,000 light years in length and surround gigantic bubbles of low-density gas. The intersections of the filaments produce regions of high density (signified by red) that are thought to generate bright X-ray clusters of galaxies.

Information supplied by: http://csep10.phys.utk.edu