Now you've hit the real crux of the ultimate question being asked by Science, "Where did it all come from?". Most of us suspect Nature is its own origin of itself. Now, that could imply that some form of space-time, or at least its primary building blocks have always been around(ala Hawking's Instanton of time), or that while the first is true a loop in time(ala Gott)brought about our present space-time structure,or that multiple vacuum states collided(ala coliding branes, etc), or you can resort to philosophy and religion and invoke the "God" equation. At the present time we've simply not got enough of a grasp on what is the proper road to a quantum gravity to actually determine exactly the right path. But the right question does start exactly at what you've just asked.