Carl Sagan, in Cosmos:
If the general picture, however, of a Big Bang followed by an expanding universe is correct, what happened before that? Was the universe devoid of all matter, and then the matter suddenly, somehow created? How did that happen?
In many cultures, the customary answer is that a "god" or "gods" created the universe out of nothing. But, if we wish to pursue this question courageously, we must of course ask the next question: Where did God come from?
If we decide that this is an unanswerable question, then why not save a step, and conclude that the origin of the universe is an unanswerable question? Or, if we say that God always existed, why not save a step and conclude that the universe always existed? There's no need for a creation—it was always here.
These are not easy questions; cosmology brings us face to face with the deepest mysteries, with the questions that were once treated only in religion and myth.