There is no god.

The Big Bang: we do not know. The Big Bang theory only explains what happens AFTER expansion begins. It does not even try to explain why the universe was in it's "initial" state of very high energy density. The Big Bang theory is based on the observation that the expansion does exist (it is real, we can't ignore it), and that the universe was hotter (and more dense) in the past than it is now. This can be observed because of the "slow" speed of light. Whenever we look very far (in distance), we also look into the past: because light took time to get here. When we look at galaxies at 1 billion light-years in distance, we are seeing the universe as it was a billion years ago. The Big Bang theory combines knowledge from many branches of science (physics, chemistry...) so that we can follow the changes that are caused as the average temperature (= energy density) drops with the expansion. When we use the theory to go back in time (trying to guess how it all started), we get to a point where the temperature is too high -- this is called the Planck Temperature. We do not know what existed (if anything) before that. What we do know is : at that temperature, matter could not exist, even the forces could not exist. Only pure energy. But we do know that matter can be made from pure energy (as long as it fits with the famous equation: E = m c^2). Since we cannot analyze the expansion before that moment, we do not know what was there. Even weirder: according to our calculations, we do not even know if "time" (as we perceive it) existed before that moment. So we are not even sure if the word "before" makes any sense when we apply it to that moment. Those are the reasons why I said that the answer is relatively easy: we do not know, and we know that we do not know, and we accept that it is possible that we will never know. However, science always tries to know. Some scientists are working on possibilities. For example, if the universe really does have more dimensions that the ones we can imagine, then there is a way to go around the problem and find a "before". But it would be in a very different type of universe. Our present universe would have "banged" from another type of universe, probably one with more dimensions (up to 11, maybe). The 11-dimension example (this is still very hypothetical): http://en.wikipedia.org... --- The real name of the theory is not Big Bang. This name was given by an adversary of the theory (Fred Hoyle) to make fun of the theory and to confuse people. It worked, because there are people who still think there was an actual explosion. No explosion. Just an expansion that is still going on (in other words, we are still inside the Big Bang). --- If the universe really is infinite (in spatial extent), then it does not need to expand into anything. It expands into itself. Imagine the present universe to be infinite. Take any distance in the universe. Go ahead, make it as big as you can. No, even bigger, don't be shy. Now double this distance. Does it still fit in the infinite universe. Yes it does. The total universe did not have to grow for your new distance to fit. You can try a similar trick with numbers: Take ALL the numbers: 0, 1, 2, 3, 4, 5... multiply them by 2 you get 0, 2, 4, 6, 8, 10... Did you create any new numbers? No. Even though the new list (the even numbers) should extend twice as far as the original list (all the numbers), it still fits inside the original list. That is the "fun" part of dealing with infinities. It may create some headaches (many people don't like dealing with infinities) but it also solves other headaches. In this case, it solves the problem of trying to figure out if the universe has to expand into something else. However, it brings in the "problem" that it was already infinite at the Planck Time.