As for what were the causing conditions that brought about The Big Bang, science has yet to determine a definitive answer. But gaps in our knowledge of the universe are constantly being filled so I suspect science will sooner or later fill in more of those gaps. But for those just tuning in, here's what we got so far:
1. Big bang
2. Formation of stars
3. Formation of heavier elements in stars
4. Formation of planets
5. Development of living matter on one at least of those planets
6. Appearance of multicellular organisms.
7. Development of vertebrates
8. Vertebrates invade dry land
9. Mammals evolve from vertebrates
10 Humans evolve from primate branch of mammals.
Stephen Hawking - Curiosity: The Questions of Life
Did a god do any of this? Maybe. But a god does not seem to be necessary.
Just because the universe exists, it does not mean that a god exists too, especially a particular god. If a god does, then we're forced to then ask where did that god come from? You're still left with the same problem. If you can say that God wasn't created but "has always existed," then why can't you take it one step further and just say that the universe wasn't created but always existed? It's special pleading to claim that gods can exist without the need for a creator of them and then say the universe cannot.
Now if you want my personal opinion, I'd bank on our living in some form of cosmic mobius strip or conveyor belt somehow fueled by chain reactions occurring within it while perpetually recycling its energy and resources as this would not violate the known laws of the universe and requires no appeals to magic.
"Which of us is responsible?
Who makes the world?
Perhaps the world is not made. Perhaps nothing is made. Perhaps it simply is, has been, will always be there...
A clock without a craftsman."
-Dr. Manhattan (Watchman)