In article ,
Martin Brown writes:
Is there any way even in principle to determine observationally if all
clusters are made of matter as opposed to some being of antimatter?

I don't think there's any way to distinguish for any one
cluster. However, if a matter galaxy falls into an anti-matter
cluster or vice versa, there should be lots of gamma rays, which
should (I expect) heat the X-ray-emitting cluster gas in an obvious
pattern. Even more spectacularly, entire galaxy clusters sometimes
collide. If a collision between two "opposite types" took place, I
expect that would be obvious.

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