The temperature of dark matter is a measure of how fast the particles making it up move, with hot referring to particles moving near the speed of light (neutrinos, for instance) and cold to particles moving much slower than that. The fate of our universe depends on how much dark matter exists—more dark matter means the universe will eventually collapse while less means an ever-expanding universe—and not whether it is hot or cold. But hot and cold dark matter play a crucial role in understanding how galaxies formed. If all dark matter in the early universe were cold, galaxies would form first and later congregate into clusters. However, the huge voids and long structures observed by astronomers would not develop. If all the dark matter were hot, large-scale structure would come first with individual galaxies forming later. But in this scenario, galaxies would have formed too late to account for observations of objects such as quasars. The most recent observations and computer simulations seem to point toward a universe with a mixture of both hot and cold dark matter.