Theoretical answers to the question have been proposed and, in many cases, explored through science fiction, and I'm going to discuss them further here. Really, the possible answers to the Fermi paradox fall into one of two categories:
- There is no other intelligent life in the universe
- They're out there, but we haven't found them yet
Each of these two categories has several sub-examples that might be applicable. In the first case, the most obvious answer might be that we are, in fact, the only intelligent life in the universe. Scientifically this answer is both unplausible and unpalatable. We know that conditions in our universe are right for the formation of life, and for that life to reach intelligence. After all, we exist (though some might argue about our "intelligence"). The scale of the universe is so incredibly vast that it's impossible to believe that intelligent life would form in one and only one location.
But maybe intelligent life develops and doesn't flourish, for one reason or another. Perhaps it gets wiped out by some natural catastrophe, such as asteroid impacts, gamma ray bursts, supernovae, or other natural disaster. Or maybe it wipes itself out through catastrophic climate change, nuclear war, engineered pathogens, self-replicating nano-machines, or through other means we haven't been clever enough to come up with yet ourselves.
Maybe they are out there, though, and we just haven't found them yet. Maybe they have such advanced technology that they could be watching us silently right now. Or maybe we're so beneath their notice that they don't attempt to communicate with us anymore than we attempt to communicate with cockroaches. It's likely that they're communication systems are so advanced that we would not detect their signals.
The final possibility I'm going to mention falls under the first category, but I saved it for last: maybe the first intelligent species to expand out into the universe has gone on a rampage and destroyed every other species that has attempted to expand off its home planet. This possibility has been thoroughly explored in science fiction, from swarms of self-replicating nanites to ginormous robotic destroyers to swarms of human-eating critters. And maybe they just haven't gotten here yet, or maybe they sweep through each sector of space every ten thousand years or so, and they'll be returning soon.
Which of these do I believe? Lacking any scientific evidence, I have to remain skeptical. The one that seems most likely to me is that they're out there, but we're not smart enough to notice. Maybe they're waiting for us to find them.
What do you think?