Are there anti-photons? (Presumably only anti-matter can make them.) Or is matter special because it matches photons gravitationally and thus answers the question of baryogenesis.
That second option leads to some very interesting implications for stars made of anti-matter. Any photons they make would be "shoved" away from the star - but where does that energy come from?
The first option means there are photons we can not detect (only anti-matter can make them, so only anti-matter can see them). That would imply there is an entire second universe out there overlayed on this one (at least photonically overlayed) that we have never seen, except gravitationally.
But during annihilation events which type of photon would be produced? Both? Randomly one or the other?
Finding that anti-matter has anti-gravity would open an absolutely enormous list of questions. It's not as simple, as OK, anti-gravity - every other part of physics would have to change as well.