A recent scientific study has estimated that the Milky Way Galaxy (which we live in) alone has a tad under nine billion Earth-like planets within it. I know many doubt the numbers, but if you understand not only the size of the Universe, but the galaxy as well then it's not so far-fetched. Now that doesn't mean all of them are similar to the present-day Earth as we know it but are in a zone close enough to a sun to support life.
You have to imagine there are quite of a few of them wondering if they are alone in the universe. I would find it very ironic that with all the sci-fi stories about aliens coming to Earth, that in the distant-future we'd instead be the visitors to a civilized world outside our solar system.