Rare Earth suggests conditions for intelligent life are extraordinarily specific and Earth may be unique. The Zoo Hypothesis proposes advanced civilizations watch us but choose not to interfere. The Dark Forest Theory suggests civilizations hide to avoid being destroyed.

Perhaps civilizations self-destruct through war, environmental collapse, or dangerous technology before achieving interstellar travel. Or perhaps the distances between stars are simply too vast — even at light speed, crossing the galaxy takes 100,000 years.