The Reticulated Universe
When simulations of the early universe are done, matter (dark and normal) collects in very large scale [tens or hundreds of millions of light years] filaments of overdensity. Galaxies and stars develop in these filaments, the voids between them are largely empty of matter. Here and there - ok, mostly there - two filaments intersect. Such intersections are the breeding grounds of galaxy clusters and superclusters. The large concentrations of dark and normal matter in such clusters draws streams of gas from the filaments whose intersection marks the cluster, with the result that the cluster is bathed in a vast sphere of superheated (10^7 -10^8 K) but very low density (10^-7 to 10^-1 baryons/cm^3)gas. If two or more such clusters are close enough, their mutual gravitational attraction will propel them into high speed (10^7 km/hr) collision. These results can be spectacular, as in the case of the "Bullet Cluster" (Formal names: 1E 0657-56, 1E 0657-558) In the picture, t...