The Zoo Hypothesis is a proposed solution to the Fermi Paradox—the glaring contradiction between the high mathematical probability of extraterrestrial life and the total lack of contact with or evidence for it. Coined by astronomer John Ball in 1973, the theory suggests that aliens are aware of Earth and are deliberately hiding from us, treating our planet like a protected wildlife preserve or a zoo.

(http://www.autoadmit.com/thread.php?thread_id=5871159&forum_id=2#49915885)