Nobody knows. Some people pretend they know, but they don’t. Physicists are working towards this though. They now know what happened a tiny fraction of a second after the big bang, and are working backwards, but they aren’t there yet.
At the moment, scientists are unsure as to what caused the Big Bang – although our understanding of the conditions closer and closer in time towards the event itself improves each day. For instance, the Large Hadron Collider in Geneva recreates some of the conditions less than a billionth of a second after the Big Bang.
I saw this interesting talk:
By the scientist Lawrence Krauss, in which he describes some of the latest scientific theories which hint that a universe from nothing is, perhaps, inevitable because of the nature of ‘nothing’ in the real world. It’s a great talk and well worth a watch.
One of the things that always blows my mind about the Big Bang is that it marks the start of both space AND TIME. Before the Big Bang, there was neither space, nor time. Amazing!
So, with regards to what caused the Big Bang – we don’t know. But we do know a lot about what happened in the time just afterwards, and you can read some of what we know here: