> Our current physics also doesn't seem to be _so_ wrong that it allows much room for technological societies to hide from our instruments.

Perhaps.

An idea I first heard from my brother a decade or two ago was that the Dark Energy expansion of the universe might be the waste product of all the aliens who figured out some way to trick the laws of physics into giving them free energy.

(Obviously that's more of a sci-fi plot device than science at this point, take it with the due level of seriousness).