Firstly, we have our 100% failure rate at what God has actually done. At first we thought he made the weather, the seasons change and the tides come in and out. Then we were told he made life in his image, and made the Earth. We've been 100% wrong about everything he's supposedly done so far. Is it too much of a leap to think we're wrong about him creating the universe, in light of this 100% failure rate?
Next, we have the latest thought in theoretical physics: the idea of membranes colliding. This is mathematically working things out very nicely. And if there's one thing we do know, is that mathematics is the universe. A mathematical model constitutes more "proof" than an unprovable deity.
Thirdly, we have no evidence to say that he did create the universe.