If there's nothing particularly unique about Earth, then there should be lots of alien civilizations in our galaxy. However, we've found no evidence of other intelligent life in the universe.

Finally, some see the silence of our universe as a paradox.

One of the underlying assumptions in astronomy is that Earth is a pretty common planet in a pretty common solar system in a pretty common galaxy, and that there is nothing cosmically unique about us. NASA's Kepler satellite has found evidence that there are probably 11 billion Earth-like planets in our galaxy. Given this, life somewhat like us should have evolved somewhere not overly far away from us (at least on a cosmic scale).

But despite developing ever-more-powerful telescopes, we have had no evidence of technological civilizations anywhere else in the universe. Civilizations are noisy: Humanity broadcasts TV and radio signals that are unmistakably artificial. A civilization like ours should leave evidence that we would find.

Furthermore, a civilization that evolved millions of years ago (pretty recent from a cosmic perspective) would have had plenty of time to at least begin colonizing the galaxy, meaning there should be even more evidence of their existence. Indeed, given enough time, a colonizing civilization would be able to colonize the entire galaxy over the course of millions of years.

The physicist Enrico Fermi, for whom this paradox was named, simply asked, "Where are they?" in the middle of a lunchtime discussion with his colleagues. One resolution of the paradox challenges the above idea that Earth is common and posits instead that complex life is extremely rare in the universe. Another posits that technological civilizations inevitably wipe themselves out through nuclear war or ecological devastation.

A more optimistic solution is the idea that the aliens are intentionally hiding themselves from us until we become more socially and technologically mature. Yet another idea is that alien technology is so advanced that we wouldn't even be able to recognize it.