In basic philosophy there is a standard argument that goes like this: Every effect has a cause, and every cause is an effect of a prior cause, and so on and so on and so on. Therefore, there are two possibilities: 1) either the universe is a cycle that has always existed, or 2) if you go back far enough, you will find the original "un-caused" cause, the prime mover, God.
Well, as it turns out, modern science has something to say about this. First, we know, for a fact, as much as any fact can be known, that the universe isn't some great cycle. The big bang is not going to end in the big crunch. The universe had a beginning and it's going to keep going and going and going, until there's no fuel left to make stars. After that, it will still keep going, filling up with dark energy as it goes, but no one is going to be around to notice. So, that rules out option #1. So, is option #2 a possibility?
Well, as it turns out, modern science has something to say about that too, though it's not proven by observation like the accelerating expansion of the universe. There is a theory called "eternal inflation." This theory says, and I'm loosely paraphrasing here, that the universe is not infinite, it has an actual size even though it is expanding, and that this universe is contained within something called eternal inflation. Further, it states that our universe popped out of this eternal inflation and went bang. Now, you have to realise that we're talking about something outside of 3D-space and before time began, so the language is a little imprecise, to say the least. Human language and reasoning doesn't really work well when we get beyond the world that we can experience. But still, the wording is good enough for our purpose here.
Now, eternal inflation is some seriously strange stuff. Basically, the rule there is that it inflates, there's more of whatever it is, and then more, and more, and more... eternally. No cause needed, just effect, lots and lots of effect. Randomly, some of this inflation doesn't inflate as per usuall and a universe pops into existence. And, here's the gotcha, the universe is a place where this inflation doesn't happen. It can't happen. The universe is a bubble of "no effect without cause" inside this eternal inflation. We exist because eternal inflation isn't happening within our universe.
Eternal inflation, effect without cause, has to stop so that space can exist, so that time can start, so that energy can cool down enough to become matter, so that gravity can clump some of this matter together, so that stars can form, so that heavy matter can fuse together, so that planets can form, so that organic chemistry can start replicating, so that life can start, so that intelligence can evolve, so that we can ask "how did this all start?" We exist because we live in a bubble, our universe, where all effects must have a cause. So, the question isn't "what was the first un-caused cause?" It's "why did the un-caused effects stop so that we can exist?"