People of faith will always answer god, even if there is sufficient scientific proof stating otherwise. You can't fault them for it- as their way of thinking makes them feel safe. The question has troubled some of the greatest scientific minds and they have left many theories to purse. Maybe you'll be one to pick up where they left off. Or, you'll simply side with the god theory. Good luck. I say that science has the answers.
To those talking about religion; not trying to be rude, but there's another category for such talk, it's above this one. Top Theories (No one has enough proof to call 'fact')
Big Bang Theory: Since stars/galaxies are spreading out, the universe is expanding like a balloon, this is backed by photos of the early universe using light reaching us now. We know what it's like now and how fast it's going, thus; how long ago it wasn't here. This is the big bang idea; nothing had to become everything. This is also why they estimate when the universe will die(too cold to work).
Creationist Theory: This varies extremely throughout time and civilization ranging from chaos from nothing(Greek). To fire and ice making a cow(Norse). To a man snapping his fingers(Abrahamic). But the idea is that something existed before the universe and intelligently designed it or was involved in its creation.
Always Here: This theory proposes that the universe always existed and always will, it's considered scientific, but lacks proof. The main problem; is if the universe has always existed, and light just keeps traveling, we shouldn't see individual stars and gases, just a blanket of light.
Im always going to side with science. The Big Bang makes MUCH more sense then god....It was a spark of ONE particle that created the universe...All it is was just a particle exploding...How hard is that? Now god.....He didn't create the universe because FIRST How did he get there in the first place? Wouldn't he need to evolve like us and be the very first one to die? I don't believe in god nor the theory that he created the universe.
Some will say the Big Bang, some will say God used the Big Bang, while others will say God. I believe that God created the universe by His word. The Big Bang lacks scientific proof and makes no sense. How does nothing explode and make everything? That is unscientific - it is religious! However, the main objection to God is where did He come from? I'll answer that too. God had no beginning, therefore He needs no cause. In other words, the Law of Cause and Effect is clear that everything that had a beginning had a cause. God had no beginning, so He had no cause. If you want some evidence for God you can go to www.icr.org/God or www.answersingenesis.org and type in "evidence of God" in their search engine. The Big Bang makes no scientific sense to me, and the idea that God used the Big Bang is unbiblical. Therefore, my answer is God spoke it into existence.
The exact answer to this is that, it was created due to quantum fluctuations, that states particles and antiparticles can arise from vacuum by uncertainty principal (actually 0=-1+1,-2+2.....-infinite+infinite)
When they can pinpoint the location of where the "Big Bang" took place, and prove there was an explosion, then I might believe it. I have no problem with the expanding universe but cosmology has left too many things unanswered.