Possible explanations for the Fermi Paradox:
• Sufficiently advanced alien civilizations would be very good at hiding. They’re doing exactly that due to threats from other sufficiently advanced alien civilizations.
• Sufficiently advanced alien civilizations get the f**k out of the cramped confines of our 4-D bubble of spacetime as soon as inhumanly possible.
• Sufficiently advanced, starfaring, alien civilizations wouldn’t communicate with EMF broadcast.
• The speed of light is absolute. Period. Their ain’t no interstellar travel, except for exceedingly rare generation ships or research vessels hopping to nearby star systems. Due to this limitation, there is zero possibility of interstellar civilization, let alone interstellar communication. The universe is filled with intelligent life. But each species is an island unto itself.
• Everything’s alive. The entire cosmos is one big organism—an ocean of emerging self-awareness. The whole damn thing is “intelligent life.”
• Blinkered species that we are, humanity doesn’t recognize intelligent life that doesn’t look like humanity. There are other intelligent life forms on Earth. We eat them, cut them up in labs, and put them in cages.
• Something really, really bad happened on a cosmic scale and wiped out all the sufficiently advanced alien civilizations in our immediate galactic vicinity. Earth is like a remote island in the South Pacific whose inhabitants somehow survived WWIII.
• There are umpteen different templates for life (including intelligent life) aside from the carbon-based model. Humanity wouldn’t recognized a typical alien life form if it bit us in the collective ass.
• The bulk of the cosmos is comprised of dark matter. We’re utterly blind to it—including any dark matter species.
• We’re living in a computer simulation. Earth is the main event. Aside from a few broad strokes, the lazy designer didn’t bother filling in the details of the so-called "Universe" outside the Earth.
• Powers unknown have burned our brains with a mental blind spot that prevents us from seeing evidence of intelligent alien life. We see it—and don’t see it. “It doesn’t look like anything to me,” is our conditioned response.
• 99.999999999% of intelligent life forms evolve into industrial civilizations which relentlessly grow—and refuse to stop growing until they devour and destroy their planetary ecologies and, after a period of cannibalism and degeneracy, destroy themselves. (Props to Piers Anthony.)
• They’re lying to us. The truth is out there, oh yes. But an evil “X-Files”-type conspiracy is covering up the evidence, blah-blah-blah.
• Earth is the alien equivalent of a game preserve. We’ve been deliberately isolated and blocked from contact with other intelligent life forms. Sure, every now and then, some alien pervert breaks through the barrier and gives a redneck in Alabama an anal probe. But, most of the time, we're on our own.