Transcript: Throughout most of the history of the universe structures have been forming under the action of gravity. Remember that the visible parts of galaxies just represent the tip of an iceberg of mostly dark matter. The way in which structure forms depends on the detailed properties of dark matter. If dark matter is cold, which to a physicist means that the particles that form the dark matter were not relativistic at the time of recombination when neutral hydrogen occurred and the microwave background photons were released, then structure forms in what’s called a bottom up way. Galaxies form first, then clusters, then even larger structures. On the other hand, if dark matter is hot, which means it was moving relativistically at the time of recombination, then structure forms in what’s called a top down way. Structure on galaxy scales is erased, and the largest structures form first. Astronomers believe there’s good evidence that the universe shows structure consistent with cold dark matter and not hot dark matter. Hot dark matter particles produce too much structure on large scales, more than is observed. Also there are no good candidates for a hot dark matter particle. Neutrinos have a tiny amount of mass but are insufficient in their mass to account for cosmic structure.