The Big Bang is the theory that tries to explain how the universe was created.
Nobody knows what existed before the Big Bang, but when it occurred - between 10 and 20 billion years ago - whatever was there exploded, and created the expansion of the universe.
Galaxies that are twice as far from us move twice as fast as our own. Since that one Bang, everything in space has been growing and moving outwards. Galaxies are moving further and further apart. The universe just gets bigger and bigger.