It is often suggested that extraterrestial life sufficiently advanced to be capable of interstellar travel or communication must be rare, since otherwise we would have seen evidence of it by now. But taking Darwin beyond the solar system may answer why

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 age of the universe and its vast number of stars suggest that if the Earth is typical, extraterrestrial life should be common. In an informal discussion in 1950, the physicist Enrico Fermi questioned why, if a multitude of advanced extraterrestrial civilizations exists in the Milky Way galaxy, evidence such as spacecraft or probes is not seen. A more detailed examination of the implications of the topic began with a paper by Michael H. Hart in 1975, and it is sometimes referred to as the Fermi–Hart paradox. Other common names for the same phenomenon are Fermi's question ("Where are they?"), the Fermi Problem, "the Great Silence"

This in turn is sometimes taken as indirect evidence for the improbability of life evolving at all in our universe.

A couple of other possibilities seem worth considering.

One is that life capable of evidencing itself on interstellar scales has evolved in many places but that evolutionary selection, acting on a cosmic scale, tends to extinguish species which conspicuously advertise themselves and their habitats.

The other is that -- whatever the true situation -- intelligent species might reasonably worry about the possible dangers of self-advertisement and hence incline towards discretion. These possibilities are discussed here, and some counter-arguments and complicating factors are also considered.

Source: arXiv