Super-civilizations that conquer entire galaxies may only exist in science fiction, a new study suggests. In the scale of possible civilizations proposed by Russian astronomer Nikolai Kardashev, highly advanced Type III civilizations are capable of harnessing the whole power of a galaxy, but a survey of the most promising 100,000 galaxies found no sign of the heat in mid-infrared wavelengths such incredible power would produce. "These galaxies are billions of years old, which should have been plenty of time for them to have been filled with alien civilizations, if they exist," the lead researcher from Penn State's Center for Exoplanets and Habitable Worlds says in a press release. "Either they don't exist, or they don't yet use enough energy for us to recognize them."
The information from NASA's WISE orbiting observatory did, however, reveal 50 galaxies with high enough levels of mid-infrared radiation to merit further study, as well as a few objects the researchers describe as "unstudied and really interesting looking." And the study definitely does not indicate an absence of alien civilizations: Kardashev III civilizations master entire galaxies, so astronomers in other galaxies using the same method wouldn't be able to detect our civilization even if we had colonized a billion stars. In fact, the lead researcher tells io9, the study suggests that if other civilizations aren't taking over whole galaxies, the Milky Way is still a very good place to keep looking for them. (Harvard astronomers think air pollution could be a sign of "unintelligent life" on other planets.)