The Fermi paradox, in brief, raises the question of why we have yet to observe evidence of extraterrestrial life. The simplest explanation would of course be that such life is extremely rare compared to just how mind-rendingly, jaw-droppingly, eye-wateringly, tooth-shatteringly huge the universe is, how hard it is to generate signals detectable across light-years of space, and what a small part of its lifespan we've been witness to, but many creators prefer to introduce a more interesting reason into their universes. Common explanations include: Examples of Invisible Aliens include: