The zoo hypothesis is one of a number of suggestions that have been put forward to explain the fact that we have so far found no evidence for advanced extraterrestrials (see Fermi Paradox). According to this idea, alien races are taking good care to avoid making their presence known to us or exerting an influence on our development (just as the Federation in Star Trek has a strict policy of non-intervention with the less technologically sophisticated cultures they encounter). Our planet and our species, it is hypothesized, may be under close scrutiny, like the animals in a zoo, without us being aware of it. One way this monitoring could be carried out would be through automatic equipment surreptitiously located on Earth or elsewhere in the solar system which relays back to its masters news of our progress. Only when we are sufficiently advanced, it is suggested, may we be let into the secret and inducted into the galactic society of beings capable of interstellar travel.
1. Ball, J. A. "The Zoo Hypothesis," Icarus, 19, 347 (1973).