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This question already has an answer here:

For instance if there was a galaxy, assume it to be made up of antimatter (isolated from other "normal" galaxies), how would we, or rather, would we be able to distinguish if it was made up of matter or antimatter? If not then why is not considered that half of the universe out there might be made up of antimatter but we think there is an abundance of matter over antimatter just because we can't detect any difference among the two?

Are there any cases in which antiparticles act differently than their corresponding particles would?

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marked as duplicate by ACuriousMind, Danu, John Rennie, JamalS, Qmechanic Nov 15 '14 at 17:57

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Neither the space between galaxies nor the space within galaxies is completely empty, but rather is filled with a diffuse gas. If there was an isolated galaxy, or region of galaxies that was made up of antimatter, there would be an interface between the two regions, and this region would be rich in annhilation events, which would be visible in X-Rays. We don't observe this (though there is still a bit of wiggle room whether we TRULY don't observe this), so we conclude that most of the matter in the universe is not antimatter.

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There's (almost) no difference between matter and antimatter, and it would be very difficult to know if a galaxy far away is made of matter or not.

The reason we know that the visible is made of matter, and not half-matter and half-antimatter is that it this were the case, then we would see a ton of events where anti-matter galaxies would smash into a matter galaxies, and that would make a hell of a bang. Since we don't see any of these events, we conclude that the univers is made of matter.

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