Invisible Aliens

"Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us."

- Calvin, Calvin and Hobbes

The Fermi paradox, in brief, raises the question of why we have yet to observe evidence of extraterrestrial life. The simplest explanation would of course be that such life is extremely rare compared to just how mind-rendingly, jaw-droppingly, eye-wateringly, tooth-shatteringly huge the universe is, how hard it is to generate signals detectable across light-years of space, and what a small part of its lifespan we've been witness to, but many creators prefer to introduce a more interesting reason into their universes.

This can serve two functions. On the one hand, it can provide the author with some excuse for referencing alien cultures, providing alien archaeological sites, and similar without actually introducing a viable alien culture. On the other, it can allow for the creation of interstellar societies that would otherwise appear implausible, especially when they've been so active in our own backyard lately that we would presumably have noticed them in reality.

Common explanations include:
 * 1) Disappearing or becoming difficult to observe (at least at interstellar distances) is a natural outgrowth of technological development.
 * 2) There's a race of aliens, generally of the Sufficiently Advanced variety, who are somehow ensuring that civilizations don't see each other.
 * 3) All the spacefaring aliens are involved in a cosmic conspiracy to remain hidden from Earth, possibly because we aren't judged to be advanced enough to know about them, or because we absolutely scare the crap out of them for some reason.
 * 4) The Weirdness Censor strikes again. The reason we haven't seen any explanation of aliens is that we're overlooking the obvious in our quest to exaggerate our own importance and significance.
 * 5) All the other aliens are on a pre-technological (or even pre-sentient) state of development. No technology = No Radio Transmissions.
 * 6) We're first. Yay us.

Contrast Absent Aliens, in which the simple explanation is taken as correct.

Anime

 * The Data Integration Entities from Suzumiya Haruhi are, according to Nagato, undetectable through our normal means. Given that they lack physical form altogether, and seem to exist only as patterns of information, this is perfectly plausible.
 * Tengen Toppa Gurren Lagann falls into the second category:

Comic Books

 * In the Ultimate Galactus trilogy by Warren Ellis, Reed Richards ponders this very question. The answer seems to be that the sky isn't filled with signals from alien cultures because Galactus ate them.

Film -- Live Action

 * In the Men in Black comics and their adaptations (and in the The Men in Black urban myth that inspired them), the title organization is expressly dedicated to making sure Earth civilians stay unaware of aliens.
 * The Alien films have alien monsters in spades, but asides from the lone, dead Space Jockey and occasional Arcturian reference, humanity does not seem to have made contact with any intelligent races. If you don't count the Predators in the alternate continuity.

Literature

 * In Marooned in Realtime by Vernor Vinge, when species attain The Singularity, they uniformly vanish from the universe, at least from the perspective of everyone else.
 * In Accelerando by Charles Stross, species all seem to build Matrioshka brains, to which they upload their personalities. The uploaded people aren't interested in interstellar travel because there's no way for them to get enough bandwidth. Everyone else is in hiding from the uploads and therefore not likely to meet each other. (It turns out that .)
 * In Alastair Reynolds' Revelation Space universe, there's As a result, all the other aliens in the galaxy seem to be either extinct or in very clever hiding.
 * In Spin by Robert Charles Wilson, planets with intelligent inhabitants get enclosed by Sufficiently Advanced Aliens in "Spin membranes", which substantially slow down time on those planets to the point where their inhabitants have no good way to make detailed off-planet observations.
 * In one of the stories in Sherlock Holmes in Orbit,
 * In The Engines of God and later books by Jack McDevitt, the galaxy seems to naturally produce things called "omega clouds" every 8000 years or so, strange clouds that are sent out in a wave from the galactic center and can look for and track excessive, clearly artificial regularity on planets—in other words, square and rectangular buildings and constructions of other shapes that are too regular and precise to appear in nature—and crash into them, sometimes catastrophically, to eliminate societies that have advanced much beyond the Stone Age. Some characters theorize that an omega cloud may have inspired the legends of Sodom and Gomorrah when it destroyed an early Earth civilization, since the relative timing fits with when the clouds should have last been through.
 * "Theyre Made Out of Meat" by Terry Bisson suggests another possible reason: the aliens found us and were too weirded out to establish contact. Can be read here.
 * The Berserker series by Fred Saberhagen has organic life threatened by the eponymous Killer Robots, who were the Doomsday Device released by the losing side of an intergalactic war. Only a few scrappy (not The Scrappy) species manage to survive, including Earth-Descended, or E.D., life.
 * The Honor Harrington novels state that humanity has, in the two millennia they have been tooling around the galaxy, encountered approximately twelve alien races. However, though there is at least one extinct race that achieved inter-stellar flight, the technology level of the other species is never mentioned. Alien races show up only as flavoring (the Sphinxian treecat, shown in almost every story) and a minor background element in two stories. For all practical intents and purposes, it is an all-human galaxy.
 * In The Metamorphosis of Prime Intellect by Roger Williams, there were hundreds of alien worlds containing intelligent life at the time of The Change, but when the title AI became omnipotent, its interpretation of the first law led it to freeze them in stasis so that they would never become technologically advanced enough to pose a threat to humanity.
 * Addressed in More Information Than You Require, as seen in this excerpt. Hodgman's theory is that Enrico Fermi himself was an alien trying to deflect suspicion. Why else would he demand his only reward be two healthy sperm whales?
 * In "The Fermi Paradox Is Our Business Model" by Charlie Jane Anders, all intelligent life-forms that evolve in the galaxy inevitably destroy themselves.
 * In Diaspora by Greg Egan, quite a few advanced civilizations developed in the Milky Way, but they all ran away into other universes to escape an imminent galactic natural disaster.

Live Action TV

 * In Star Trek, the Prime Directive prohibits the Federation from interfering with aliens until they've discovered interstellar travel; according to Star Trek: First Contact, the Vulcans had a similar rule before the Federation was founded, which presumably "explains" why we haven't seen any Vulcans yet in real life.
 * In Doctor Who, the Shadow Proclamation, basically a galactic police force, has rules about how alien life forms are not allowed to make contact or interfere with worlds without a minimum amount of technology- not that most the aliens seem to care about that, as they really like trying to take over/destroy/control Earth for some reason. It was this rule and groups like UNIT and Torchwood which kept the public learning about alien existence until around 2008 when

Video Games

 * In the old Lucasarts game Zak Mccracken And The Alien Mindbenders, the Skolarians claim (in a recording) that they left for another world millennia ago, though not before littering Earth and Mars with hidden artefacts should the Caponians attempt to invade. The Caponians, on the other hand, mostly hide with the aid of nose glasses and tall hats. Since they're successfully dumbing down Earth's population significantly, they actually manage to pull it off... until the heroes interfere, that is.

Web Comics
""There is no Fermi Paradox! Every time space faring aliens make it to Earth, the cows get them!""
 * Last Res0rt has a few planets with civilizations they're waiting to see "grow up" before they walk in and help them the rest of the way; it's implied that Earth was let in only a couple millenia ago, and Adharia's planet, Sevru, is still a few centuries off.
 * However it's also implied that the Talmi were only recognized as a sapient species a few centuries ago. Also, most of them are Jewish.
 * In The Inexplicable Adventures of Bob, Earth is located in the Nemesite Empire, but the Nemesites have designated our world as a nature preserve and have tried to leave us fairly undisturbed, just like animals in a national park. It's recently been revealed that another reason is that
 * Sam Starfall (a highly visible alien), in Freefall, has an interesting explanation:

Western Animation

 * An episode of South Park explained that we don't see aliens because all life on Earth was put there by them as part of one really long-running alien reality TV show called—you guessed it -- "Earth".
 * Oddly enough, the Mysterons of Captain Scarlet are not Invisible Aliens; they're just aliens that happen to be invisible.

Real Life

 * Journalists of more sensationalist outlets know the public's interest in the possibility of alien life. They take unpretending basic research and turn it into gonzo stories about the imminent discovery of sapient, technological extraterrestrials. We've discovered more planets (or more earthlike planets) around other stars, therefore E.T.! We've found microbes growing in the most hostile conditions ever, therefore E.T.! We've found new deposits of water or basic organic chemicals in space, therefore E.T.! New theories say there are even more stars in the universe than we thought, therefore E.T.! Our biological research suggests it's easy for life to begin, therefore E.T.! This leads to laymen becoming increasingly disillusioned: We seem to get ever closer to confirming the existence of alien life, yet we never actually get there.