Fermi paradox

The Fermi paradox is the conflict between the lack of clear, obvious evidence for extraterrestrial life and various high estimates for their existence.

There have been many attempts to explain the Fermi paradox, primarily suggesting that intelligent extraterrestrial beings are extremely rare, that the lifetime of such civilizations is short, or that they exist but humans see no evidence. This suggests that at universe time and space scales, two intelligent civilizations would be unlikely ever to meet, even if many developed during the life of the universe.

"If life is so easy, someone from somewhere must have come calling by now."