"The big bang was about a cause for the universe."

It only seems that way if you mistake "universe" as used by physicists to mean existence rather than the entities within the natural universe. The big bang theory is about the explosion of something (like a "cosmic egg") into the array of astronomical entities that physicists refer to as the universe. It is not about the creation of existence out of non-existence.

Maybe quoting the entire sentence would help: "These concepts ["explanation" and "causality"] have no meaning if removed from the context of existence, and the theist's demand for a cause or explanation for the universe reduces to nonsense." (emphasis added)