According to most scientific experts, given the vastness of the universe, the likelihood of extraterrestrial life is high. In fact, the odds of alien life elsewhere in the universe has gone up considerably in the last 10 to 15 years as we have discovered that the preconditions for life (a planet in habitable temperature zone with water and the presence of the chemical building blocks for life) are not that uncommon in our galaxy.

The growing likelihood of extraterrestrial life obviously begs the question of where are all the aliens? Unfortunately, the answer to the question is quite complex and there are half a dozen competing theories to explain the apparent lack of aliens.

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“Rare Earth hypothesis”

One of the most reasonable explanations for the lack of proof of aliens to date is the “rare Earth hypothesis.”

This explanation suggests that habitable planets such as Earth are rarer than we think for reasons currently unknown to us. That is, even if there are billions of planets in the habitable “Goldilocks zone”, maybe gamma rays or asteroids or other unknown problems stopped life from developing.

George Mason economics professor Robin Hanson suggested the possibility of a “great filter” that lies “along the path between simple dead stuff and explosive life” that is very difficult or even impossible to move beyond. If the “filter” happens at the early stages of life, that would explain why no other planet has produced life.

Some “rare Earth hypothesis” supporters such as paleontologist Peter Ward and astronomer Donald Brownlee argue that the development complex, intelligent life on this planet may even be a once-in-a-Big Bang occurrence.

Maybe nearly all alien life is non-technological

“The absence of strong radio beacons, television broadcasts, robotic spacecraft, obelisks on the moon — all of those absences add up to give us the suggestion that our galaxy is not teeming with technological life,” Geoffrey Marcy, an astronomer at Cal Berkeley, explained to The Washington Post in February.

So maybe the reason we have found no proof that other life that exists because that life has not developed communications technology.

Along these lines, NASA’s chief scientist Ellen Stofan said earlier this year that humans would indeed discover life beyond Earth in the next couple of decades. But we won’t find “little green men,” she noted at a public panel in this spring.

“We are talking about little microbes,” she said.

Another possibility, of course, is that extraterrestrial life is trying to communicate with us, but their signals haven’t reached us yet or they have and we don’t recognize them as a communication.

Aliens too advanced to care about humans

One other possibility is that highly advanced aliens may have already visited Earth, but before humans could document the visits. many who believe this theory say they may even have left signs of their presence behind.

The explanation goes on to suggest that the aliens just weren’t interested in humans because we are not advanced enough or too violent or Earth doesn’t have resources they need, so they just left.

Earth is part of an isolated “zoo” of primitive species

A final possible explanation for the lack of extra-terrestrial life is the “Zoo hypothesis,” suggested by academic John Ball.

“Extraterrestrial intelligent life may be almost ubiquitous,” notes the abstract to Ball’s 1973 paper in the journal Icarus. “The apparent failure of such life to interact with us may be understood in terms of the hypothesis that they have set us aside as part of a wilderness area or zoo.”