Consider a hypothetical particle which, like the photon, is chargless and massless. However, unlike the photon, it is absorbed by nothing and reflected by nothing. Its refractive index through any material is always 1. It does not interact with anything by any means other than the gravitational effect caused by its energy. Let us call this a 'dark photon'. I am sure it does not exactly fit in current definitions of dark matter or dark energy, it is just something inspired by them.

Let us now assume that sometimes, for some particles, a particle/antiparticle annihilation can yield these dark photons rather than normal photons. Is there any way to distinguish this situation from one in which the particle and antiparticle just disappear into nothingness, with their mass and energy removed from the universe in violation of mass energy conservation?

I, with my admittedly quite Newtonian view of gravity, cannot see one. For example, an observer 'looking' at the particle and antiparticle gravitationally would 'see' a point particle with their total mass-energy at their centre of mass-energy. This would remain true after they have been converted to dark photons, except that the gravitational force would disappear once the dark photons have moved past the observer. The time taken for this would be r/c, if r is the distance from the observer to the centre of mass-energy. The same is true after they disappear into nothingness, the observer would still feel the force for a time r/c, since the effects of gravity are not instantaneous. The same is of course also true for normal photons, except that this case can be distingushed from the previous two by detecting the photons, or using some material to slow them down. But how could dark photons be distinguished from a violation of mass-energy conservation? Is it possible? If not, does that mean mass-energy conservation prohibits the existence of such particles, even though they don't actually violate mass-energy conservation?