Big Bang Theory

The now favoured theory of the origin of the Universe first put forward in 1927 by Georges Lemaitre. It considers that the universe came into existence some 15,000~20,000 million years ago when a highly compressed and intensely hot ball (primeval atom) of matter exploded.
The universe has been expanding ever since and will continue to do so.

Expanding universe a concept in astronomy that considers the universe as a whole to be getting bigger. There is evidence for it in the observation that most galaxies are receding from us, with the farthest ones travelling the fastest