Galaxies are thought to be composed of matter. But what if they are composed of anti-matter? Can it be shown that they are composed of normal matter? What kind of observation could distinguish between the two? Would observations made on galaxies be different if they are composed of anti-matter. Is it even possible that anti-matter galaxies exist?
So, there have to be dividing surfaces between regions containing (eventually) anti-matter only and regions containing only matter. On these surfaces (thin volumes), a reaction between both types of matter can occur. Matter and anti-matter discombobulate, with the result that gamma rays will flow into the surrounding space. But are there enough rays to be visible on Earth? Can there be other observations that hint at the segregated states of matter (if real)? Can we ever be sure that aren't separate regions, thereby confirming the theoretical assumption that there can't be separate regions?
What is needed to detect? For sure, the detection has to be mediated by photons, or gravity. So, will there be a difference in the photons produced by a normal galaxy and an anti-one? Is it possible maybe that anti-particles interact differently with the Higgs field, giving a different mass galaxy? Which should be observable.