How big?

I was thinking about the universe and how big it is. I'm told there are over 350,000,000,000 galaxies...if there are that many galaxies I can't imagine how many stars there are or how much space there is between them. So the thought occurs to me that there are only two possibilities when it comes to how big the universe is and both are beyond my comprehension. Either it goes on forever, or it ends somewhere. How can there be no end to something? But if it ends somewhere, what's on the other side of the end of the universe? Something else, I guess. But what would that be, and how far does that go!