Pondering this question: Casimir effect and negative mass and, in particular, the response of John Rennie "as the mass of any bound system is slightly less than the mass of its parts" I thought: "This statement hold true even for gravitationally bounded system?"
If we have m1 and m2 so close that their gravitational interaction is not negligible, the system composed of m1 and m2 has a total weight less than m1 + m2 ?
If so, if we consider 2 galaxies, since each galaxy is composed by masses (M) and gravitational binding energy between those masses (E), it is possible that the gravitational attraction between those 2 galaxies is proportional to (M1 + E1/c^2) * (M2 + E2/c^2) ? (with E1 and E2 < 0)
Googling "Gravitational binding energy as alternative to dark matter" i have found this that should be relevant for the discussion: An explanation for dark matter and dark energy consistent with the standard model of particle physics and General Relativity