The Fermi Paradox, named after physicist Enrico Fermi, is the contradiction between one factor with high probability yet has no evidence of existence, such as aliens. The universe is full of the building blocks for life similar to humans, and has existed long enough that it is plausible that contact could have been established by now, so what's up? Enrico Fermi ran the Drake equation (used to estimate the number of alien species open to contacting humans), and found that humans should have encountered an alien, and yet, nothing.
It might be the most boring theory and maybe not a likely one, but it explains why UFOs have not been discovered by anyone except poor photographers and not actual astronomers, and why no communication has taken place. All the pieces are there, but not a soul in sight. Imagine buying all the drinks and snacks for a party, sending out infinite invitations, and no one showing up. In Fermi's own famous words, “Where is everybody?”