Yes, ultimately I *believe* in gravity. People find evidence of God's existence; others find evidence of gravity's existence. The difference - as I'm sure you are aware - is that one passes the scientific method; the other doesn't or won't submit to it. Then again, dogma finds a way into many non-religious things (hang out with raw vegans and you'll see that in action).
Playing my own Devil's advocate - one can argue that a man-derived scientific method is insufficient to test for the existence of God; after all, if I bake a cake and it decides I don't exist, the cake doesn't disappear.
One thing I think you have wrong -- God wouldn't be inside the universe. God created it. I suppose - going back to the cake analogy - God could bake himself into the cake. But he would still be an exo-cake being, and would have existed before the cake.