I have never been uncomfortable with the idea of the universe having a limit, because it seems like all physical things must have limits. (Based on our definition of life it seems like a universe could almost be a living thing.)

This strays from the topic at hand, but I have always wondered what was beyond the universe. Is our universe one bubble in a space of other bubbles (universes)? Has our universe been expanding and contracting for all time? Did God put our universe into being? Is there any stock to be held in string theory?