The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high likelihood of its existence. As a 2015 article put it, "If life is so easy, someone from somewhere must have come calling by now."
The argument can be summarized like this: it seems likely that intelligent life has evolved elsewhere in the universe -- the universe is a big place. It took a very long time for this evolution to occur in the only case we know of -- it took us billions of years. One can reasonably assume that some supposed alien intelligent life out there will evolve more quickly, others less. Statistically, we should assume that we are most-likely about in the middle in terms of our rate of evolution -- it would be unexpected for us to be the very fastest-evolving intelligent life in the universe. It takes time to travel between the stars, but as best we can tell, not very long compared to the kind of time required to evolve -- once a civilization is able to do travel in space, we would expect it to spread, and do so quickly compared to the time required to evolve. So if one guesses that maybe half of alien intelligent life evolved more quickly than we did, a lot of it should have had a lot of time to spread throughout the universe by now.
But we have seen nothing that appears to be alien intelligent life on Earth or elsewhere. How can this be?
There are some proposed answers to the paradox that are a bit disturbing.
It is the nature of intelligent life to destroy itself
This is the argument that technological civilizations may usually or invariably destroy themselves before or shortly after developing radio or spaceflight technology. The astrophysicist Sebastian von Hoerner stated that the progress of science and technology on Earth was driven by two factors—the struggle for domination and the desire for an easy life. The former potentially leads to complete destruction, while the latter may lead to biological or mental degeneration. Possible means of annihilation via major global issues, where global interconnectedness actually makes humanity more vulnerable than resilient, are many, including war, accidental environmental contamination or damage, the development of biotechnology, synthetic life like mirror life, resource depletion, climate change, or poorly-designed artificial intelligence. This general theme is explored both in fiction and in scientific hypothesizing.
We are right about at that point in technological development ourselves. We cannot yet travel to the stars, but we can travel in space, and reaching the stars does not seem to present fundamentally unsolvable challenges. If that answer is the correct one, then we would expect such a destructive event to occur to humanity before long.
It is the nature of intelligent life to destroy others
Another hypothesis is that an intelligent species beyond a certain point of technological capability will destroy other intelligent species as they appear, perhaps by using self-replicating probes. Science fiction writer Fred Saberhagen has explored this idea in his Berserker series, as has physicist Gregory Benford and, as well, science fiction writer Greg Bear in his The Forge of God novel, and later Liu Cixin in his The Three-Body Problem series.
A species might undertake such extermination out of expansionist motives, greed, paranoia, or aggression. In 1981, cosmologist Edward Harrison argued that such behavior would be an act of prudence: an intelligent species that has overcome its own self-destructive tendencies might view any other species bent on galactic expansion as a threat. It has also been suggested that a successful alien species would be a superpredator, as are humans.