If God didn't create everything, who did?
This argument is a result of a theist not being able to imagine that it was not in fact, a being that made the universe.
The word "who" makes this a complex question: the person asking the question presupposes that the universe was created by a person (a "who" rather than a "what"). A better question would be "How did the universe come into existence?"
The question can also be seen as a false dichotomy that assumes that either 1) God created the universe, or 2) some other being did so, and ignores the fact that both of those explanations might be wrong.