There are many theory's suggesting different things. Some think it came from a parent universe in which we are the creation of another universe. Others believe our universe has a timeline, and in expands contracts and then expands again from that. Another is that we are just one of many universes in a multiverse. I even believe there is one that suggests that every black hole contains another universe in it self so we could just be the inside of a black hole. So when a black hole formed a white hole was formed that acted as a valve and shot out all of the mass in our universe. One of the most popular is string theory which has the possiblity of what could be before the bigbang if it was prooved true. The thing is if we can't see it people will speculate, any of those theory's could be correct but because we can't go into a black hole our leave our universe or see any evidence it shrinks we can't know for sure.
Scientists are looking for if another universe might have smashed into ours leaving a mark. Determining if dark energy is real or not could make some theory's impossible like the falling back together. But I don't think it is truly possible to ever know for sure unless we can check out some black holes, see the universe shrink, or noticing a universe colliding with ours. A good source for other theories is howstuffworks.com type in what was before the Big Bang.