FIGURE 1. Simulated cosmos.

The density distribution of matter in a slice of the computational volume of the Millennium Run model, showing large clusters with densities 1,000 times the mean density of the Universe (yellow); a 'cosmic web' of filamentary structures 10 to 100 times denser than the mean (purple); and the mostly empty regions (black), often called voids, which contain less than 10% of the mean density of the Universe. The white square shows the size of the computational volume for a full hydrodynamic simulation that would use up the same computational resources as the Millennium Run. (Figure courtesy of Volker Springel.)