Big Bang

The scientific theory of the origin of the universe as an explosion of space and matter, starting from an enormously dense and hot state at some finite time in the past. (At this point the estimate is 14 billion years ago). The central idea is that the observed redshift of the galaxies (Hubble's law) shows that the galaxies are receding from each other, which implies that they were much closer together in the past. - Exerpt from Wikipedia. Information held under GNU Free Documentation License.