A hypothetical universe. The picture shows a computer's guess of how gas in the universe was distributed billions of years ago, at redshift 3, when the universe was only a quarter of its present age. This hypothetical universe was reconstructed by the GC3 team (Grand Challenge Cosmology Consortium) using supercomputers at the National Center for Supercomputing Applications. A cube of this universe was cut out and is displayed here, with each side being about 8 million light-years across. The colors represent varying temperatures of the hot gas. At the bottom, a single slice through this cube is displayed, with helium abundance superposed with a wire mesh. The small structures appearing here give insight into the past and present structure of the intergalactic medium, the matter inhabiting the regions between galaxies and clusters of galaxies.