Youre thinking of nothing within something, thats the problem.
I think you're thinking of nothing like.. when a ball is not in position A there is nothing in position A. How does A go from nothing to ball without a cause?
And thats good reasoning; but the absence of ball in A is the difference. There can't even be an absence; because the universe is all there is. Thats time space everything. It simply is, and its not like there was some before or after etc, it is just is.
I'm pretty hopeless trying to type quickly a concept none of our brains can truly 'get', because its totally contrary to our experience of the world.