Before the Big Bang, the universe was (theoretically) compressed into a small, infinitely dense and immeasurably hot space. Think of 200 billion suns times 200 billion galaxies compressed into an area as small as an atom or something. Under normal circumstances, matter compressed to such extremes creates a black hole from which nothing can escape. However, the Big Bang theoretically involves the creation of the universe [i.e. the creation of space time itself.]
So not only did matter explode, but space and time exploded outward in ways that would be impossible in our universe today. Before the Big Bang, space and time did not exist in the same way that it does now [just using grammar to understand a time before time is awkward]. The universe before the universe was a time before time, and in a space before space. Or something else.