I've heard this assertion before about the glow from annihilation events. So I started looking into what we should be looking for.
Of course you are only going to get this effect between am/m pairs, not m/m or am/am.
Proton/antiproton annihilation results in a pair of gamma rays of around 938MeV. These may be red-shifted if the galaxies are far away, so we could be looking for a spectrum maybe between 90-900MeV.
What about the intensity of this radiation (if it were there) what would we expect? Would all the gas have mixed and annihilated aeons ago, or do you expect to see gas streaming out of the galaxies still? If so, at what rate?
Finally, looking at various observations from gamma ray observatories, these are currently at low resolution and show some radiation that may overlap with the expected spectrum. So I don't think the matter is settled, unless you can come up with specific research sources that can demonstrate the negative result with some certainty.