The Drake equation calculates the probability of currently active, technologically advanced, extraterrestrial civilizations in our galaxy. Drake's lowest estimate was one thousand. One recent estimate is fifteen million.

The Fermi paradox is the apparent contradiction between the Drake equation and the lack of evidence of extraterrestrial civilizations; either from radio waves, artefacts, or visits. Fermi asked bluntly "Where is everybody?"

There are several hypothetical explanations for the Fermi paradox.

One is that extinction events such as large meteorites, massive volcanic eruptions, or astronomical events such as gamma-ray bursts may be common and would periodically destroy intelligent life before the species would be able to develop the technology to communicate with others.

Another explanation is that intelligent extraterrestrial life has developed interstellar travel but does not contact Earth to allow for its natural evolution and development. This zoo hypothesis would be adopted as a galactic norm in order to not interfere with more primitive civilizations.

Conversely, UFO conspiracy theories claim that extraterrestrial civilizations have visited or are already here unacknowledged. No evidence has been presented to substantiate those claims.

Yet another explanation is that technological civilizations may usually or invariably destroy themselves - before or shortly after developing radio or spaceflight technology - through nuclear war, poorly designed artificial intelligence, or climate change.

Using the Drake equation, a new study published in the journal Astrobiology has concluded that over cosmic history (not only currently active), in the observable universe (not only in our galaxy), advanced civilizations have developed ten billion times.

Ett galleri i natten 2018, Mitt Möllan. Malmö, Sweden.