This is the hardest question of all, and right now NOBODY has a good answer to it. All known laws of physics break down at the time of the big bang, and all we can do at the moment is speculate. We DO know that at this time gravity (which is due to the warping of spacetime) is subject to fuzzy quantum mechanical effects, and this suggests that space and time themselves become poorly defined concepts. It might not even make sense to ask questions such as "what happened before the big bang" or "what created the big bang" because the very notion of time might be breaking down at that point. Currently the best hope of attacking this problem is a very abstract theory called "string theory" or "M-theory", which is an attempt to unify all the fundamental forces of nature, including gravity, into one single quantum theory. However, nobody can yet write down the mathematical equations which describe this theory, and there is no way to use it to tackle this most interesting of questions. If somebody tells you that they know what happened at the time of the big bang, or what caused it, don't believe them. We still don't know!

If the Big Bang theory is true, no one, to my knowledge, has offered a scientific theory to answer that question.Since science deals with what can be observed, the Big Bang theory explains the observed behavior of the universe. But, since the Big Bang itself was not observed, a good theory to explain what created the Big Bang cannot be offered.

Good question. I don't think anyone can answer that, assuming that the question even makes sense. Time did not exist before the Big Bang, so it doesn't make sense to ask about things "before", since there is no "before" the Big Bang.

But that's not exactly the same thing as what you're asking.