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You're spot on with the gamma producing zones. We don't see any of those, and that's why we don't think any galaxies are made of antimatter.

https://www.scientificamerican.com/article/how-do-we-know-th...

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I thought there were signs of gamma radiation surrounding galaxies. Perhaps the gas filled void between galaxies is antimatter while the galaxies are matter. Perhaps antimatter is repelled by both types and regular matter is attracted to both types. This might lead to a gaseous cloud pushing things apart and clumps of regular matter. It would also allow some configurations to accelerate and violate conservation of momentum, so it doesn't seem likely.

To me there are more questions than "does antimatter fall up?" There are 4 interactions to test.

If anti-matter falls "up" wrt ordinary matter, but still self-attracts, then there wouldn't be any gamma producing zones.

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