If you haven't heard of the Fermi Paradox, it goes something like this: Given the vastness of the universe and the probability that implies of life evolving elsewhere, how come no alien civilization has ever gotten in touch? We have found many exoplanets in the brief time we've been looking. Surely there must be someone else out there who, like us, desperately wants to find others?
Since it was posed in 1950 by Enrico Fermi, there have been a range of answers, from the benign to the absolutely terrifying. One is that there simply hasn't been enough time yet. Alien civilizations may prioritize, as we do, searching for techno signatures, which we simply haven't been broadcasting for long enough. On the other end of the spectrum, it could be that the tendency throughout the universe is for civilizations to destroy themselves before they reach sufficient advancement to make contact.
An early-ish explanation, proposed by John Allen Ball in 1973, is the Zoo Hypothesis. In this idea, aliens may be aware of us but are hiding themselves from us.
"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."
Just as we set aside areas of land as nature reserves and leave uncontacted tribes uncontacted, advanced civilizations may choose to allow us to evolve on our own and watch our progress, as we watch animals in a zoo. As civilizations become sufficiently mature – technologically or politically – they would make contact. Like on Earth, these previously uncontacted civilizations would "eventually be engulfed and destroyed, tamed, or perhaps assimilated".
This would imply that there was a ruling civilization/collection of civilizations, who all agreed to respect the zoos, much like the "prime directive" in Star Trek.
"So, generally speaking, we need consider only the most technologically advanced civilizations," Ball wrote, "because they will be, in some sense, in control of the universe."
Unfortunately, like with many other solutions to the Fermi Paradox, it doesn't provide us with a way to test it out. Essentially, it's like not knowing if your house is empty, or is crammed full of people who are so technologically advanced you could be looking right at them and seeing nothing but a lamp.
"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."
Ball did not like the idea, describing it as "psychologically unpleasant" as it would be nicer to think aliens would want to contact us if they knew we were here.
"However the history of science contains numerous examples of psychologically unpleasant hypotheses that turned out to be correct," he concluded.
Of course, it's not altogether clear that we would want to make contact even if we could. According to the Dark Forest Hypothesis, if there is even one civilization out there hell-bent on destruction, it might make sense for all other civilizations to keep quiet, in case we should accidentally attract their attention.