The Big Bang theory says how the Universe began and is evolving.
This theory explains two facts about the Universe : it is gradually expanding and cooling. In the 1920s, Edwin Hubble found that galaxies far from our own Milky Way are moving away from us. He concluded that the whole Universe must have been expanding. That means that at one stage the Universe must have come from a single point.
We also know that the Universe is cooler now than in the past. The Universe began as an infinitely compact fireball.
The Big Bang describes how this fireball grew to form all the stars and planets we see around us now. During the Big Bang space was first created and then stretched.