Artist's impression of the Big Bang, the titanic explosion which cosmologists believe created the Universe. Recent estimates suggest the Big Bang occurred about 15 billion years ago. The theory of the Big Bang is based on the fact that the visible universe is still expanding, & on the discovery of background radiation - the so-called microwave background" - that is believed to be the remnant from the primordial explosion. In this picture, the Big Bang is portrayed at a time when giant clouds of gas (white spots) have already started condensing into what will later become galaxies."