Did it really happen?
The Big Bang Theory is just a way of explaining how the universe started. In history there have been discoveries in astronomy and in physics that prove that the universe had a beginning. It has been proven through standard theories that the beginning of the universe came into existence as a "singularity" 13.7 billion years ago. A singularity is a small source which exist in the cores of black holes. It was thought that it started as a very small very hot singularity. The three famous astrophysicists Steven Hawking, George Ellis and Roger Penrose still can't find out what the singularity existed within. It wasn't in space so they don't know what it was surrounded by before the big bang.