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Fermi's Question, also called Fermi's Paradox, is named after physicist Enrico Fermi (lived 1901-1954) who asked in a conversation with fellow physicists in the summer of 1950, "Where are they?" when talking about extraterrestrial life. Consider these facts and reasonable assumptions:
Given those facts and reasonable assumptions, why haven't we been visited or at least been contacted yet? There's a strong likelihood that we are not the only technologically advanced intelligent civilization that has arisen in the universe, SO: where are they? There is no scientifically credible proof that we have been visited---see the "Pseudoscience vs. Science" essay for more about UFOs as alien spaceships and what is required for credible proof of extraterrestrial life visitation. Fermi's Question remains.
There are many possible ways to answer Fermi's Question, so the list of answers given here is not complete but it does give the more popular answers.
We really are alone. Even if extra-terrestrial life is present, it is most likely microbial (like bacteria or archaea). Life on Earth was solely microbial for most of the history of life (i.e., the past 3.5 to 3.8 billion years). Complex life (multi-cellular organisms that don't take a microscope to see) like plants and fungi did not arise on Earth until about a billion years ago with land plants not appearing until just 550 million years ago. Animals appeared in the sea soon after the plants and developed on land within a few tens of millions of years after the plants. Complex life on Earth has approximately another 500 million to 1 billion years before conditions on Earth become too hot too exist due to the ever-brightening sun. The window for complex life on Earth is just 1.0-1.5 billion years wide while the window for microbial life is six to seven billion years wide because microbial life is tougher and can handle much more extreme conditions than can complex life. The fragility of complex life coupled with the apparent lack of evolutionary advantage to being intelligent, means that fact/assumption #5 above is wrong---beings with enough intelligence to build radio telescopes and space ships are extremely rare. Furthermore, if the planet is completely covered in water, any intelligent creatures wouldn't be able to create the electronic technology necessary for communication or travel across space (electronics require dry land). So many things had to go just right in the history of life on Earth for us to develop. We are the first technologically advanced civilization to arise in the Milky Way. If ET's exist in other galaxies, they are too far away for us to know about, so we are effectively alone.
Technologically-advanced civilizations self-destruct. All civilizations that develop the capability for space travel and radio telescopes also develop chemical, biological, and nuclear weapons of mass destruction and destroy themselves or they pollute their home planet so bad that they can't survive beyond that phase in their technological development. We are the only technologically advanced civilization existing now because the other civilizations have come and gone.
Technologically-advanced civilizations capable of interstellar travel are destroyed by the most advanced civilization in existence. The malevolent ET civilization considers others an existential threat and destroys other emerging civilizations before they have a chance to compete. A civilization capable of harnessing the huge amounts of energy needed to warp spacetime for interstellar travel would not have to use laser or phaser weapons like you see in the movies. Instead they could create a small black hole near the emerging civilization's planet and totally obliterate the planet or create a targeted biological weapon that wipes out the emerging civilization while leaving the natural resources of planet unharmed for extraction by the more advanced civilization. Similar to this is the possibility that when interstellar civilizations come into contact with each other, they destroy each other unwittingly via some sort of virus or other contagious disease.
A technological or biological obstacle makes interstellar travel impossible. There is some obstacle we are not aware of that makes interstellar travel impossible or impractical. We are already aware of the great biological challenges in sending people to Mars and the obstacles that have to be overcome with lengthy journeys among the planets, including ill-timed solar flares and coronal mass ejections. Stars are hundreds of thousands of times farther apart from each other than are the planets in a planetary system and stars do provide a magnetic bubble/shield around their planetary systems. Perhaps journeys in interstellar space would take much too long for civilizations to want to voluntarily take the risk. However, these considerations don't prevent civilizations from attempting communication.
There is a "Prime Directive" of non-interference for all emerging civilizations such as ours. The Galactic civilization is watching us while deliberately avoiding contact until we have become stable and mature enough to make contact, like the "Prime Directive" of Star Trek. Instead of watching us, perhaps the ET civilization is ignoring us and has installed some sort of mechanical sentinel in our solar system or just beyond the boundary of our solar system that will be activated when we finally reach a stage in our space exploration capabilities to cause the sentinel to send a message to the ET civilization (like the black monolith of Arthur C. Clarke's "2001: A Space Odyssey"). Perhaps the ET civilization has its own agenda and simply doesn't want to contact us. We humans like to think we are special and worthy of ET's attention but that could be conceit on our part. Some propose that we should look for other signs of these advanced civilizations besides leaked or directed electromagnetic signals. Perhaps we could detect infrared emission from huge mechanical structures such as Dyson spheres (structures enclosing a star to capture all of the star's light) or arrangements of astronomical objects that appear to have been set up artificially (though this runs the risk of being a "ET of the gaps" type of problem where we blame some bizarre natural phenomenon on ET instead of trying to understand the natural cause).
The Galactic civilization is so advanced that we can't recognize it. Our situation could be like an ice-age human coming across a field scattered with smartphones and thinking they were blocks of obsidian. ET's technology is so far outside of the realm of our experience and intuition to see that the artifacts around us must be artificial. It is much more likely that civilizations coming into contact with each other will be at vastly different levels of technological capability than what you see in the movies---all those aliens look so much like humans with masks and strange make-up and their spacecraft controls look like variations of our fighter plane cockpits! Given the millions or maybe even billions of years difference of when life arises on the different planets in our galaxy, perhaps our situation is more like humans compared to ants. Both have social structures and can manipulate their immediate environment but ants don't recognize us unless we mess with their nest. Perhaps the ET civilizations have been trying to communicate with us in ways that would seem to be obvious to even primitive creatures on ET's home planet but like a human flashing Morse code signals with their flashlight above the ant nest, we don't notice the symbolic language being used. Maybe the other civilizations are so far beyond us that they just don't notice us and our leaked radio broadcasts haven't had enough time to reach them. Maybe we are so different from ET culturally and technologically that we don't recognize them and they don't recognize us.
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last updated: August 5, 2022