The Big Bang Theory is the scientific model for the origin of the universe, it was born out of the observation that other galaxies are moving away from our own at great speed as if propelled by an explosive force.
The theory states that about 13.7 billion years ago all the matter and radiation of the observable universe was compressed into a hot, dense mass just a few millimetres across. This state is theorised to have existed for just a fraction of the first second of time after which a massive blast allowed all the universe's matter and energy to escape. In the instant (a trillion trillionth of a second) after the Big Bang the universe expanded from a few millimetres to something astronomically vast and has continued to expand , though more slowly in the billions of years since.
Cosmic microwave background radiation received from all parts of the universe is thought to be the heat left over from the original explosion.