... or, why haven't we met any aliens? From The Conversation:
Last week, scientists announced the discovery of Kepler-186f, a planet 492 light years away in the Cygnus constellation. Kepler-186f is special because it marks the first planet almost exactly the same size as Earth orbiting in the “habitable zone” – the distance from a star in which we might expect liquid water, and perhaps life.
What did not make the news, however, is that this discovery also slightly increases how much credence we give to the possibility of near-term human extinction. This because of a concept known as the Great Filter.
The Great Filter is an argument that attempts to resolve the Fermi Paradox: why have we not found aliens, despite the existence of hundreds of billions of solar systems in our galactic neighbourhood in which life might evolve? As the namesake physicist Enrico Fermi noted, it seems rather extraordinary that not a single extraterrestrial signal or engineering project has been detected (UFO conspiracy theorists notwithstanding).
This apparent absence of thriving extraterrestrial civilisations suggests that at least one of the steps from humble planet to interstellar civilisation is exceedingly unlikely. The absence could be caused because either intelligent life is extremely rare or intelligent life has a tendency to go extinct. This bottleneck for the emergence of alien civilisations from any one of the many billions of planets is referred to as the Great Filter.The possibilities are that (a) we've already made it through the Great Filter (i.e., that it is some evolutionary or catastrophic bottleneck that, because of Earth's unique combination of being in just the right place, with the right axial tilt, with a large moon to stabilize the planet, and large outer planets to clean out most of the debris in the solar system, etc., hasn't destroyed or prevented intelligent life and civilization), or (b) it may be something that only impacts technological civilizations which we have not yet encountered (e.g., a hostile alien intelligence, competition from an artificial intelligence, etc.). Of course, there is another possibility, though, which is some power (God, an alien civilization) that prohibits contact.
Or, maybe some of these UFO incidences actually do involve extraterrestrials, and there is no Great Filter. It may just be our conceit that an alien intelligence wants to contact us. Perhaps they have no more interest in stopping and meeting with our leaders than we have in stopping and meeting the mayor of a small rural town we drive through as we travel from city to city.