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This question addresses the first part of my question. In that it tells us how we would theoretically differentiate antimatter galaxies from regular matter galaxies.

How would we tell antimatter galaxies apart?

How would one detect antihydrogen in the universe?

How can we detect antihydrogen?

However I am also interested to know if any experiments have been done to iron out the possibility of some random distant galaxies being made of antimatter.

Qmechanic
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Huw Evans
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Space isn't a perfect vacuum. Intergalactic matter has very low density, but there is a lot of it. If some galaxies were antimatter, in theory gamma ray astronomers would see matter-antimatter annihilation radiation from their interaction with the intergalactic medium. We don't see it.

John Doty
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  • But how do we know that intergalactic medium is made up of matter? – Pradyuman Apr 18 '23 at 06:53
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    @Pradyuman at some point there would be a boundary between matter and anti-matter areas. Even if sparse, there would still be a lot of radiation from annihilations. – rghome Apr 18 '23 at 07:53