This is both a good question and a bad question. The rules of physics as we understand them break down as we come closer and closer in time to the event which is called the big bang. Traditionaly, cosmologists and physicists have agreed that there is no practical and testable way to determined what happened BEFORE the big bang which would have caused the big bang.
However, that does not mean there are not explanations out there. Certainly there are good ideas (not even hypothesis, as they can not be tested) which look at a diety of some kind being the ultimate cause of the big bang. Some very good efforts have been made weaving religion, philosphy and science have been developed.
One of the hopes of String Theory is to open up a way of determining what happened which began the big bang. However, to date, the explainations offered are not testable which makes them no better (or worse) than any other explanation which can not be tested. If it isn't testable, it isn't really science-- it is just an opinion. The good news is that there may be, over time, the developement of the theory in such a way as to provide a methodology for testing its assertions. One well respected modern scientists who believes this to be true is Stephen Hawking. His latest book The Grand Design outlines this very idea.