If you haven't heard of the Fermi Paradox, here it is in a nutshell: Given the high probability that alien life exists (bearing in mind the vastness of space and planets within habitable zones keep being found), why has nobody got in touch yet? If there are so many other civilizations out there – possibly far more advanced than we are because of how long the universe has dragged on – surely at least one would send out messages or probes, or desperately search for signs of life like we are?

Answers to the paradox range from optimistic to downright frightening. It could be that we simply haven't been looking long enough, nor emitting our own traceable signatures for aliens to find us yet. It could be that no aliens will ever be able to make contact with other species, destroying themselves long before they get to the kind of tech required to do so.

One of the more optimistic explanations is the Zoo Hypothesis. First set out by MIT scientist John Allen Ball, it suggests that aliens exist and are aware of us, but are quietly watching, like you would observe the animals in a zoo.

"Among currently popular ideas about extraterrestrial intelligence, the idea that 'they' are trying to talk to us has many adherents," Ball wrote in his paper. "This idea seems to me to be unlikely to be correct and the zoo hypothesis is in fact the antithesis of this idea."

"I believe that the only way that we can understand the apparent non-interaction between 'them' and us is to hypothesize that they are deliberately avoiding interaction and that they have set aside the area in which we live as a zoo."

The theory relies on certain assumptions, such as that few civilizations out there in our galaxy are at the same development point as us. This might be a reasonable assumption, given the short time span in which human civilization has developed. Instead, for the theory to work, there would be primitive life out there, plus advanced civilizations which have survived long enough to be at development levels "perhaps comparable to what will be on earth a few million years hence". 

"Analogy with civilizations on Earth indicates that most of those civilizations that are behind in technological development would eventually be engulfed and destroyed, tamed, or perhaps assimilated," he explains. "So, generally speaking, we need consider only the most technologically advanced civilizations because they will be, in some sense, in control of the universe."

Ball points out that even at our own level of technological progress, we set aside areas for natural development (from nature reserves to un-contacted peoples who we deliberately leave alone).

"The perfect zoo (or wilderness area or sanctuary) would be one in which the fauna inside do not interact with, and are unaware of, their zookeepers."

According to modifications to the theory by later scientists (and science fiction writers, of course), aliens could be allowing our natural evolution to take place, much like the Prime Directive in Star Trek prevents the Federation from interfering with the natural progression of alien species. 

It could be that hypothetical advanced civilizations wait until we are ready to make contact (such as in Contact) or until they have passed some sort of technological or political threshold (e.g. they have stopped attempting to destroy each other, or other species).

Annoyingly, the only real way we'd know if the theory is (still possibly) correct is by the process of elimination.

"The zoo hypothesis predicts that we shall never find them because they do not want to be found and they have the technological ability to insure this," Ball writes. "Thus this hypothesis is falsifiable, but not, in principle, confirmable by future observations."

He describes this as pessimistic and psychologically unpleasant, preferring to believe that aliens would in fact get in touch. Tucked away in Ball's paper on the Zoo Hypothesis is a small tweak that he describes as "morbid and grotesque": the Laboratory Hypothesis. In this version, aliens do not speak to us as we are part of an experiment that they are conducting on us. 

"We may be in an artificial laboratory situation," he writes. "However, this hypothesis is outside the purview of science because it leads nowhere, it immediately calls into question the premises on which it is based, and it makes no predictions."

By contrast, we could – and for a reasonably cheap price – at least attempt to contact our zookeepers, as physicist João Pedrode Magalhães proposed in 2016.

"I propose to send a message using television and radio channels to any extraterrestrial civilization(s) that might be listening and inviting them to respond," the author wrote. 

"Even though I accept this is unlikely to be successful in the sense of resulting in a response from extraterrestrial intelligences, the possibility that extraterrestrial civilizations are monitoring us cannot be dismissed and my proposal is consistent with current scientific knowledge. Besides, issuing an invitation is technically feasible, cheap and safe, and few would deny the profound importance of establishing contact with one or more extraterrestrial intelligences."