The Universe couldn't have had a beggining as such because no something could have come from nothing. For somthing to be created there must be energy. There has to be an exchange of energy for physical things to have ever been formed. Which suggests there was never nothing, and ther has always been somthing. This seems to lead on to the chain of thought that the universe runs in a cycle. One universe is created from the remains of the last. It builds up and builds up up to a point where it gets too large and explodes. From the remains of the energy from this universe the next is created. But this poses the question how was the first of these universes created. This is a question that will never be answered. No one knows and there is no way we can know. maybe the Universe never was created. It just always was and always will be.
If the Universe is expanding what is it expanding, what is it expanding into? Maybe there isn't just the one Universe maybe we are part of a Multi-verse (some may have seen this film, but I think it's theory is sound). If so does this prove the existence of alternate dimensions? again we cannot know. If there is a Multi-verse is there more than one of these too? if so how long does this go on for?
O.K I'm ansking more questions than I'm answering here. But my point is this. Yes since the beggining of time (in a planetary sense), man has wondered about the Universe and forever, or for however long we are here, man will always wonder. You are not the first to think such mind boggling thoughts and you wont be the last. But unless our time on this Earth is up it seems likely that you will not come up with any sort of answer to these questions. The fact is for a few days you'll think and muse and worry, and then it will go away, and again will be restored the blissful ignorence of humankind.
EDIT:
As for the existence of God, here's a theory for you all that ought to make some sense. God can't exist because he was created in the minds of man. And as such if he was created by man, he can't have possibly created man. If he did exist he would not be in the form of a human being, as many would veiw him, as creating us in the image of himself would be vanity one of the 7 deadly sins. If "he" had a gender, that would again be vanity having his gender replicated into half the population of the world. God is a theory, much like the big bang, in ancient Greek and Roman theology, whenever there was an entity that the people did not understand, a God was created to explain it. The people didn't understand how the sky was held there so Titan was created, Thunnder made no snse so a god of thunder was created, The movements of the sun across the sky, a God for that was created as well. As these things have been explained, the God's have dissapeared from religion. The God we all speak of is just the same as these just the question that he is suposed to explain is the ultimate one, why are we here?... because God put us here. This is why all religions are stupid because if the question were ever to be answered sientifically, all religions would be nul-un-void. So just because we don't know the answer to these questions doesn't automatically prove the existence of God, and in fact if we look back at History, if anything it proves his non-existence. And now my head a splode.