A Universe in a Box . ... It is one computerized 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. ... A cube of this universe was cut out and displayed, with each side being about 8 million light years across. ... 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 . ...