The most commonly asked question concerning the Big Bang Theory is - "what caused it? What was before the Big Bang?"
The honest answer - which is commonly provided within this forum - is that we do not know. The theory itself simply does not address anything prior to the Big Bang event. In fact, it is fairly clear that - since time and space are intimately connected - as determined by Dr Einstein over 100 years ago - in fact both time and space were originated by the event. This would mean that time did not exist prior to the Big Bang. Mathematically speaking, this makes perfect sense. Intuitively, it is difficult to comprehend. It means that there is no point in discussing what occurred before the Big Bang, because there is no such thing as "before" the Big Bang - at least in terms that can be easily described.
The second most commonly asked question regarding the Big Bang is - "How can something come from nothing?" Well, without making any claims that this is in fact what occurred, we know that something comes from nothing all the time at the quantum level of reality. If something was not constantly coming from nothing down there among the electrons and the quarks, then your computer would not function. On the average, mass / energy are conserved. On an individual quantum basis, that rule sorta fades away with the rest of the quantum weirdness that takes place all the time, although we cannot see it happening. If this stuff was easy, we would all be nuclear physicists.