Space is Colder Than We Hoped express
Any device, from a toaster to a smartphone, heats up when operating. This is the inevitable result of energy conversion — there's always a heat trace left, like a hot stove burner after cooking. The law of entropy applies to galaxies too: if advanced civilizations live there, reprocessing the light of entire stars, they should 'glow' with infrared heat — like a kitchen where something is stewing around the clock.
Astronomers pointed the heat-sensing WISE telescope at thousands of neighboring galaxies. Using photometry — measuring brightness in different shades of thermal radiation — they compared the actual glow with what ordinary old stars produce. Natural 'hot spots', like dust around black holes, were excluded in advance. The result: not a single galaxy showed a clear excess.
The strict conclusion: no more than 0.016% of nearby galaxies can contain civilizations that turn at least a fifth of starlight into heat. The universe seems suspiciously cool, like a kitchen after the residents have moved out, leaving us in solitude.
The temperature of 300 Kelvin they were looking for is a comfortable 27°C. Remarkably, the human body radiates about as much heat as a 100-watt light bulb. From a galactic civilization, however, they expected a glow hundreds of trillions of times more powerful.
The search for engineering structures of alien races is a popular sci-fi plot. For example, in Ivan Efremov's novel 'Andromeda Nebula', the characters find a vanished civilization by its residual radiation.