Wiki-hopping: Fermi Paradox
Fermi Paradox: Where Are They?
The Fermi paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for, or contact with, such civilizations.
The extreme age of the universe and its vast number of stars suggest that if the Earth is typical, extraterrestrial life should be common. … Fermi questioned why, if a multitude of advanced extraterrestrial civilizations exist in the Milky Way galaxy, evidence such as spacecraft or probes are not seen. … Another closely related question is the Great Silence—even if travel is hard, if life is common, why don’t we detect their radio transmissions?