Much like god, that just pushes the question up one level
A necessary thing, by definition, is its own explanation. "I am what I am" etc.
From there, classical theists attempt to connect that necessary thing to what you'd commonly understand as God.
The answer to Big Bang is that before it, there was no time itself, so there's no notion of "before" (or if you wish, it's an error in the question itself that assumes there was "before").