We don't know. There's several theories. One says God started it. Another says probability always exists, so regardless of how inconceivable it is an entire universe could have sprung out of nothing, it [i]had[/i] to happen eventually. I follow 'brane theory. Essentially this: According to String Theory our universe exists in more than the 4 dimensions we are aware of. Depending on the particular version of String Theory it can be anywhere between 5 and several hundred, although the 5 more popular versions all say 10 or 11. We can't see or interact with them, because they're either very small, or curled. This 11 dimensional plane is known as a membrane, or 'brane. Many versions of 'brane theory suggest that there are an infinite number of 'branes, stacked one on top each other, like sheets of paper in a stack. The 'branes oscillate, or vibrate. Once every few billion billion years two 'branes collide, unleashing a massive amount of energy into both 'branes. It's this energy that kickstarted the Big Bang. On the quantum level matter and energy are the same thing, and pretty much all models of the early universe (Pre CMB) have matter condensing from energy as the universe expanded and cooled.