The Dink Network

There's no direct proof of the Big Bang, which is why it's just a theory. But we can tell from the fact that galaxies are moving away from each other that the universe is expanding, so from that we infer that the universe started from something incredibly tiny, and then "exploded" to cause this expansion. Or something like that. As for what caused the Big Bang, there are theories that say that it happened because two universes collided, but there's no way to prove something like that yet.

IMO there is more substance in this than a god went "poof, the universe exists now." But there's no proof for this either.