I am aware that there are many similar questions here about this in this site, but most answers concentrate on negative inertial and gravitational energy. My question is more specific.
QM together with special relativity tells us that we can generate interactions in which same charges repel if using spin one force carriers (photons). And same charges attract if using spin 2 carriers (gravitons?). I do not see anything wrong if, by symmetry you could have two mass charges, negative and positive. Of course, I am assume they work only as charges, so the inertial mass will still be positive, same too with the active mass of general relativity responsible for curvature. Here the principle of equivalence would work only for positive masses, or best, for the absolute values of the masses regardless of sign.
So is there any reason in which general relativity forbids a gravitational force of opposite sign? (do not call it gravity is that helps). In such a scenario I suspect that same charges will clump separately from positive charges and repel each other. So perhaps we have in nature galaxies made of negative matter whose dynamics would be otherwise indistinguishable from galaxies with positive matter.
Is this scenario plausible or does GR forbids it?