Galaxies are not scattered randomly throughout the universe. Instead, they group into stringy filaments that span hundreds of millions of light-years. How did such structure evolve from the bland primordial soup that followed the Big Bang? New clues are coming from an ambitious mapping project, the Sloan Digital Sky Survey, which has measured the distance to galaxies halfway across the observable universe. Daniel Eisenstein is director of the Sloan Digital Sky Survey III (SDSS-III) and a member of the National Academy of Sciences.