Crazy theory here. What if intelligent species have a tendency to destroy themselves when they reach some kind of technological achievement? Maybe they're experimenting with warp drives, or some kind of teleportation tech and create a black hole(or some other cataclysmic event) destroying their worlds?
Other theories
-They don't communicate with radio waves(already mentioned)
-The evolution of intelligence is the exception rather than the rule. Without an asteroid hitting the Earth and shaking things up, there would probably still be giant dumb reptiles running around killing each other.
-Interstellar travel simply isn't possible in a practical sense. Without warp drives good luck actually getting anywhere fast.
Crazier theories:
-Aliens are out there and just don't want us to know about them. They might (understandably) think our species isn't ready yet to join the 'galactic union' or whatever.
-Our government knows about aliens and hides it from the public.
Craziest theory: We're all simulations in the supercomputer of a super advanced society. They're simulating the evolution of intelligent life.
-Other random ideas.
-The existence of life is extraordinarily special and life has never occurred anywhere else(or in very few places in the universe). For all we know the nearest intelligent aliens might be in another galaxy.
-Religious people are right, God created life here and there are no aliens.
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The
Fermi Paradox (aka "Where is everybody") is about the absence of evidence for extraterrestrial intelligence. Copied from the linked Wikipedia site:
5 Hypothetical explanations for the paradox
5.1 Extraterrestrial life is rare or non-existent
5.2 No other intelligent species have arisen
5.3 Intelligent alien species lack advanced technology
5.4 It is the nature of intelligent life to destroy itself
5.5 It is the nature of intelligent life to destroy others
5.6 Periodic extinction by natural events
5.7 Inflation hypothesis and the youngness argument
5.8 Intelligent civilizations are too far apart in space or time
5.9 It is too expensive to spread physically throughout the galaxy
5.10 Human beings have not existed long enough
5.11 Humans are not listening properly
5.12 Civilizations broadcast detectable radio signals only for a brief period of time
5.13 They tend to isolate themselves
5.14 They are too alien
5.15 Everyone is listening, no one is transmitting
5.16 Earth is deliberately not contacted
5.17 Earth is purposely isolated (planetarium hypothesis)
5.18 It is dangerous to communicate
5.19 They are here undetected
5.20 They are here unacknowledged
I like the one (I read somewhere) that says eventually they figure out how to jump to another, better 'dimension' if you will, which is why they never come here. It's used to explain reported sightings where they appear then disappear (i.e. not with a cloaking device per se that makes them undetectable/unobservable but still 'here'). Eventually, if we don't annihilate ourselves, the theory goes we too will discover how to do that.
