This, logically, leads to the question "What was before the Big Bang?".
There are several theories about this; some suggest that we grew out of another, older, universe ("rapid expansion" amongst others); another that it collapsed into a single point from the previous incarnation (big bounce theory).
There are others, all explaining some of the observable evidence but nothing has satisfied all the anomalies yet.
As more evidence becomes available, science will continue to evolve and change it's theories until it arrives at a solution, just as it has throughout history and not just sit back and say "God dunnit"