Nothing.
Before the big bang, it was all one tiny little piece of mass that consisted of a black hole. In a black hole, time does not exist. Scientists have concluded that the black hole exploded, just for no reason at all. It just did.
For those who think "God made it explode", there was no time that existed before the big bang, therefore there would be no time for a god to perform such action.
I'm about to offend like 30 people.
Discuss. Quoted this off of Curiosity from Discovery Channel