The Big Bang theory says that the universe came into being from a single, unimaginably hot and dense point (aka, a singularity) more than 13 billion years ago. It didn't occur in an already existing space. Rather, it initiated the expansion—and cooling—of space itself.
How did the universe began?
The Big Bang was the moment 13.8 billion years ago when the universe began as a tiny, dense, fireball that exploded. Most astronomers use the Big Bang theory to explain how the universe began. The matter that spread out from the Big Bang developed into everything in the universe, including you.