A K3 civilization is one that has diverted a significant fraction of the stellar output of a galaxy to its own uses. They have to dump their waste heat somehow, and that's visible in the far to mid infrared. So we can compare the IR emission of galaxies to look for oddballs with a lot of IR.

https://www.centauri-dreams.org/2015/04/16/g-hat-searching-f...

“Our results mean that, out of the 100,000 galaxies that WISE could see in sufficient detail, none of them is widely populated by an alien civilization using most of the starlight in its galaxy for its own purposes. That’s interesting because 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. Either they don’t exist, or they don’t yet use enough energy for us to recognize them.”