The Fermi Paradox

The Fermi paradox, or Fermi's paradox, named after physicist Enrico Fermi, is the apparent contradiction between the lack of evidence and high probability estimates^[1] for the existence of extraterrestrial civilizations.^[2] The basic points of the argument, made by physicists Enrico Fermi (1901–1954) and Michael H. Hart (born 1932), are:

• There are billions of stars in the galaxy that are similar to the Sun,^[3][4] and many of these stars are billions of years older than the Solar system.^[5][6]
• With high probability, some of these stars have Earth-like planets,^[7][8] and if the Earth is typical, some may have developed intelligent life.
• Some of these civilizations may have developed interstellar travel, a step the Earth is investigating now.
• Even at the slow pace of currently envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years.^[9]

According to this line of reasoning, the Earth should have already been visited by extraterrestrial aliens. In an informal conversation, Fermi noted no convincing evidence of this, leading him to ask, "Where is everybody?"^[10][11] There have been many attempts to explain the Fermi paradox,^[12][13] primarily either suggesting that intelligent extraterrestrial life is extremely rare or proposing reasons that such civilizations have not contacted or visited Earth.

Source: Wikipedia