Extensive galaxy surveys have shown that structures resembling sheets and filaments characterize the galaxy distribution. We can study the formation of the structures in the galaxy distribution by using computer simulations, which follow the motion of a set of particles that interact with each other gravitationally. These particles represent the distribution of matter in a finite volume of the Universe. Figure 1 Figure 1 shows four snapshots from a simulation carried out on the Cray parallel supercomputer at the Garching Computer Centre, which followed the motions of 17 million particles within a volume of 5 million cubic megaparsecs.
Time runs from left to right and from top to bottom.
Each panel is a slice 170 megaparsecs on a side and 16 megaparsecs thick. Bright spots indicate regions where the matter density is several orders of magnitude larger than the mean. Filaments and underdense regions are found in the simulated matter distribution, much as in the real Universe. The Universe is mostly filled by material which does not emit light. Simulations like the one shown in Figure 1 only follow the motion of this dark matter.