Assuming such beings work on the same physics we do, they need massive amounts of energy to run a galactic civilization; and they have the same sources available that we do: Stars; and for extra credit black holes.
Putting a dyson sphere around a star changes its energy output, shifting it far into the infrared. Putting a matrioshka brain around a star is even more dramatic a change--but it still has the same gravitational effect on other stars. But no, unfortunately that doesn't explain the effects of "dark matter."
Putting a dyson sphere around a star changes its energy output, shifting it far into the infrared. Putting a matrioshka brain around a star is even more dramatic a change--but it still has the same gravitational effect on other stars. But no, unfortunately that doesn't explain the effects of "dark matter."