On the one hand, you can argue that since ex nihilo nihil fit, the universe, in one form or another, must be eternal and must have always existed. One potential problem with that point of view is the question of whether actual temporal infinities can really exist. On the other hand, the traditionally preferred answer has been: God did it. Of course, this answer doesn't explain anything, since it leaves the question of how even God could have created something (everything, in fact) from literally nothing, unsolved. Yes, God of the gaps argument, not a real solution.
But the question remains: where did the universe come from? And in the following fascinating, amusing and thought-provoking conversation, evolutionary biologist Richard Dawkins and theoretical physicist Lawrence Krauss tackle this and other equally important and interesting questions. Of particular interest is Krauss's distinction between the way scientists understand "nothing" from the way theologians understand the same word. It's a really interesting way of turning the question around on the believers. I wonder what you'll make of it...
Hope that leaves you with an intellectually satisfying weekend.