The Big Bang Theory proposes that the Universe exploded into existence about 15 billion years ago. It started out unimaginably small, bright, hot, and dense but has been expanding ever since. (It now has a radius of about 15 billion light years.) During the course of this expansion some of the mass of the Universe has condensed to form countless billions of stars. These stars are concentrated in galaxies, of which there are ten billion in the known Universe. These galaxies are grouped into clusters, which are themselves grouped into superclusters, separated by vast distances in empty space.