I have no idea what created the Big Bang. Not a clue. And I'm reasonably confident that nobody else does either, or ever will. I say this not as a defeat but as a starting point — because the honest admission of not knowing is, philosophically speaking, considerably more interesting than a confident answer that doesn't withstand examination.
Let's establish what we do know. The Big Bang is the prevailing cosmological model for the origin and development of the universe. The theory holds that approximately 13.8 billion years ago, all the matter and energy that exists was contained in an extraordinarily hot, dense singularity — and then rapidly expanded. We have good evidence for this: the cosmic microwave background radiation, the large-scale structure of the universe, the observed expansion, and the relative abundances of light elements all point to it. The Big Bang is not speculation. The physics is solid.
The question of what caused it, however, is a different matter entirely — and this is where things get philosophically treacherous.
The standard theistic move is to invoke the First Cause argument: everything that begins to exist has a cause, the universe began to exist, therefore the universe has a cause — and that cause is God. It is a structurally clean argument, which is partly why it has survived for centuries. But it has problems.
The most obvious: if everything that begins to exist has a cause, what caused God? The usual response is that God is eternal — uncaused, self-existing, outside of time. But then we're allowing that something can exist without a cause. And if we allow that, the question becomes not "does the universe need a cause?" but "why is God a more satisfying uncaused thing than the universe itself?" The argument grants its own exception and then declines to apply that exception consistently.
The deeper problem is more interesting. Our concept of causation — of one thing bringing about another — is built entirely from experience within the universe. Causes precede effects in time. But time itself emerged with the Big Bang. To ask "what happened before the Big Bang?" or "what caused it?" may be as malformed a question as asking "what is north of the North Pole?" The framework we're using to ask the question may not apply at the boundary conditions we're asking about.
Stephen Hawking and James Hartle proposed a model in which the universe has no boundary in imaginary time — no definable "beginning" in the way the question implies. This doesn't mean it has always existed in any conventional sense; it means the question of its origin may need to be asked differently, or not at all.
"If the Big Bang theory is true, then the universe had a beginning. If the universe had a beginning, it had to be created. Not necessarily by an old geezer with a long white beard who intones like John Gielgud, but there had to be some catalyst of some kind. Intelligent or not, it created the universe and is therefore the creator. I would appreciate it if you could come up with a good argument against this."
Mark's point is well-put — and he's right that the first cause argument doesn't require a personal God, just some catalyst. My response then and now: the argument establishes, at most, that something preceded or caused the initial conditions of the universe. It tells us nothing about the nature of that cause. An impersonal quantum fluctuation, a prior universe in a cyclical cosmology, a mathematical necessity — these are all candidates. Invoking a personal God is an additional claim that requires additional justification.
Furthermore, even granting a first cause, you then face the same regress. If event X caused the Big Bang, what caused X? Whatever answer we reach, the question can be asked again. This is not a rhetorical trick — it's a genuine problem. The first cause argument terminates the regress by declaration rather than by resolution.
My actual answer to the question in the title: we don't know, may never know, and should be suspicious of anyone who claims otherwise with confidence. What we can say is that the universe exists, that it had a beginning of some description, and that the physics of that beginning sits at the edge of what our current models can describe. That edge is genuinely interesting — more interesting, I'd argue, than papering it over with an answer that merely restates the question at a higher level of abstraction.
The universe doesn't owe us an explanation for itself. But it seems willing to let us keep asking, which is almost as good.