maybe some of the residents found a use for all that dark matter and have exhausted the supply
We’re thinking alike here, PIF... I was thinking maybe Kardashev had it wrong: A Type III civilization isn’t a civilization that has harnessed all the energy of the stars in a galaxy, but one that’s harnessed all the energy of the dark matter in a galaxy (don’t ask me how you convert dark matter into energy!). You want proof that Type III civilizations exist? Look no further than DF2.