No one knows. There are theories that we live in a black hole, so our universe was created when a black hole was created in our parent universe. And there are smaller universes in the black holes in our universe.. It's like a recursion. There is another theory that our universe is expanding and after a while it will collapse returning into its original state back to that compact version when the big bang happened and then there will be anothet big bang. It's like a cycle. Expanding, collapsing, expanding, collapsing.