Given that the universe has been around for billions of years, and humans as a species have only been on this planet for a few thousand years, I think it's reasonable to assume that there has been plenty of time for other civilizations to develop. After all, we managed to pull it off quickly, even with all the unnecessary conflicts we've had.

Also, with our massive output of electromagnetic radiation, we would not be very difficult to notice if some form of life with the technology to detect it passed close enough to us. We already have the technology to detect a similar output of radio waves.

But as the Universe is so infinite, and we have only been here for such a small fraction of time, it is very likely that we just haven't been found yet. It takes light over 100 billion years to travel across the universe, and we can't come close to those speeds.