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).
Time travel, if possible, could result in some extremely strange situations.
A physicist working on inventing the time machine is visited by an older version of himself. The older version gives him the plans for a time machine, and the younger version uses those plans to build the time machine, eventually going back in time as the older version of himself.
The bootstrap paradox is the opposite of the classic grandfather paradox: Rather than going back in time and preventing oneself from going back in time, some information or object is brought back in time, becoming a “younger” version of itself, and enabling itself later to travel back in time. One then has to ask: How did that information or object come into being in the first place?
The bootstrap paradox is common in science fiction and takes its name from a short story by Robert Heinlein.
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.