Certain people believe that there is a God, who brought our Universe into existence. They cannot explain God's nature, however they believe that he exists.
Other people believe that the Universe simply exists. That it was not created, that it just exists. Logically that can only mean that it always has existed, or that it just popped into existence, from nowhere.
Forget things like "the Big Bang", because even if we accept that it took place then something must have started that. Either God, or the Universe pre Big Bang. (Or we're just back to the "simply popped into existence with no cause" solution.)
At the end of the day neither side can currently prove that they are right. It's just down to individuals to make up their own mind. This can be done in two ways (or some combination of them). By logical deduction or by faith (that goes for either side, faith is not solely the province of those who believe in the God solution).
Personally I go with the God scenario. For the simple reason that it is the simplest solution, I am a fan of "Occums Razor" as you may have noticed.
Please also bear in mind that the existence of God does not pre-suppose an afterlife for us, or a Heaven, or anything like that. We may simply be a self-aware part of the Universe God created. That's another story entirely. It's not an "if you believe God created the Universe, then you must accept everything else" situation, much as some people may try to convince you it is.