Big Bang. Artist's impression of the Big Bang, the titanic explosion that cosmologists believe creat- ed the Universe. Recent estimates suggest the Big Bang occurred about 12 billion years ago, although the figure is uncertain. The Big Bang theory is based on the fact that the visible universe is still expanding outwards from a central point, and on the discovery of background (microwave) radia- tion that is thought to be an afterglow" of the explosion."