Search for Galactic Civilizations

A 2015 paper in Astrophysical Journal reported the results of G-HAT (Glimpsing Heat from Alien Technologies) survey. Using data from NASA's orbiting WISE observatory, Penn State astronomers searched for so called Type III civilizations in 100,000 galaxies beyond our Milky Way.
In the 1960s, Kardeshev, a Russian astronomer, classified hypothetical civilizations based on their energy use. A Type III civilization consumes the entire energy output of its galaxy. Such a civilization is called K3 (Kardeshev 3) by the researchers.
The G-HAT study found no such civilization in any of the 100,000 galaxies surveyed. While some ET civilizations might span an entire "island universe," none are utilizing all the energy from the stars in it.
The researchers did find unusually high levels of mid infrared radiation in 50 of the galaxies. While a K3 civilization doesn't appear to exist, a K 2.8 or K 2.9 civilization, utilizing 1-10% of available galactic energy, may be possible. Further research will try to determine if the mid level IR energy results from natural processes or is artificial.
I have three comments:

• First, any galaxy with a K3 system would be difficult to detect or recognize as a galaxy in the first place. Capturing and utilizing all stellar energy would require covering all galactic stars with Dyson spheres, or similar structures. To an outside observer, the stars would be blotted out. Moreover, Dyson spheres, if efficient, would give off only minimal infrared energy, which is invisible. There might be some K3 civilizations which have simply proved undetectable, or are not recognizable as  galaxies.

• Second, if the observed levels of mid infrared are natural, why are they so rare? Fifty galaxies out of 100,000 is a mere 1/20 of 1%--only one galaxy in 2,000. Abundant mid IR would be understandable if it were due to a spreading intelligence. Colonizing and exploiting much of a galaxy is a daunting challenge. It probably isn't particularly common. Natural causes, on the other hand, should be. And what is the cause? Extensive dust lanes, comets and solid bodies that soak up stellar radiation and reradiate it as IR? All galaxies probably have that. Perhaps some galaxies have more than others if for some reason there was more nucleosynthesis, hence a greater percentage of nonradiating mass (except for IR) formed of heavier elements. But I don't know what circumstances might've led to that.

• Third, what we see is not the current situation in remote galaxies but the distant past. If a galaxy is 100 million light years distant we're seeing it as it was when Earth was dominated by dinosaurs. If ETs 50,000 LY away could look at Earth, they'd see no evidence of civilization whatsoever. It is possible that civilizations, even some approaching K3 status, arose in the period between the remote past, as seen now, and the actual present in distant areas of the Universe.