Is it possible that an alien race has simply covered that galaxy with dyson spheres? That life is more common than we thought - or perhaps more rapidly advancing once intelligence is hit - and most galaxies are "dark" from their interference?
Identifying one of the many infrared sources as a Dyson sphere would require improved techniques for discriminating between a Dyson sphere and natural sources. Fermilab discovered 17 potential "ambiguous" candidates, of which four have been named "amusing but still questionable". Other searches also resulted in several candidates, which are, however, unconfirmed.
We often seem to assume we know "everything", and we often find out there is more to learn. It would be an interesting universe if we found dark matter to be ancient civilisations. Using up much of the mass and energy (even the infrared).
Oh stop. This is such an unlikely avenue of inquiry that it just distracts from the conversation.
If at some point we figure out that the laws of thermodynamics do not apply, that would be such a profound change in how we interact with the universe that speculation over how to best search the night sky for intelligent life would be the last thing on most people's minds for quite some time.
We would quickly find ways to harness quantities of energy that would fry us to a literal crisp today due to conversion losses, and that would be such a breakthrough that rather than look for signs of life in nearby systems, we could just go there and look around.
It should be detectable, but the Ĝ Infrared Search[1] didn't find any in the WISE[2] data. Jason Wright talked about this and related topics in a SETI institute video[3].
