Physicist Stephen Webb has been collecting solutions to the Fermi paradox: 

‘Many of Webb’s collected hypotheses suggest that aliens live where we’re not looking, talk how we’re not listening or resemble something we haven’t sought out. Maybe the aliens like to send messages or signals using neutrinos, nearly massless and barely-there particles that don’t interact with normal matter much, or tachyons, hypothetical particles that fly faster than light. Maybe they use the more-conventional radio or optical transmissions but at frequencies, or in a form, astronomers haven’t sought out. Maybe a signal is sitting on data servers already, escaping notice. Maybe the extraterrestrials subtly alter the emissions of their stable stars, or the blip-blip-blip pulsations of variable stars. Maybe they put something big — a megamall, a disk of dust — in front of their sun to block some of its light, in a kind of anti-beacon. Maybe their skies are cloudy, and they consequently don’t care about astronomy or space exploration. Or — hear Webb out — perhaps they drive UFOs, meaning they are here but not in a form that scientists typically recognize, investigate, and take seriously….’

— Via Astronomy.com

…or maybe we’re simply alone.