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Given that antimatter galaxies are theoretically possible, how would they be distinguishable from regular matter galaxies?

That is, antimatter is equal in atomic weight and all properties, except for the opposite reverse charge of the particles, identical to regular matter. Hence a star composed of antimatter hydrogen would fuse to anti-helium in an analogous way to our own Sun, and it would emit light and radiation at the same wavelengths as any regular matter star and would cause the same gravitational forces for planetary systems to form as in any other star system.

Hence, what would be a telltale sign if you were observing a galaxy made up entirely of antimatter?

Also, is there any evidence for that half of all galaxies are not made of antimatter -- while general theories currently assume that there is an imbalance of matter over antimatter in the universe, then what is the rationale for not assuming that there is in fact an even balance between the two?

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An antigalaxy could have negative gravitational mass in theory - leading the question whether that has been modelled/simulated somewhere –  Tobias Kienzler Nov 4 '13 at 15:08
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You're right - for isolated galaxies, there is no obvious way of discerning whether they are made of matter or antimatter, since we only observe the light from them. But if there are regions of matter and antimatter in the universe, we would expect to see HUGE amounts of radiation from annihilation at the edges of these regions. But we don't. You could also make the case that galaxies are well-separated in space, and there's not much interaction between them. But there are plenty of observed galaxy collisions even in our own small region of the universe, and even annihilation between dust and antidust in the intergalactic medium would (probably) be observable.

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The intergalactic medium is sparse by any terrestrial definition of matter, but given the large volume of it, there's a lot of it. Like Ben said, if there were any region of space dominated by anti-matter, the IGM would have to switch "polarity" as well. The boundary or transition area between matter and anti-matter regions would produce spectaaaaaaaaacular fireworks, which we just don't see anywhere in our visible universe. –  Andrew Jul 11 '11 at 14:44
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Would spectral absorptions give any indication that you're looking at antimatter? ie, does anti-hydrogen have a different absorption line to regular hydrogen? –  Rogue Jul 11 '11 at 16:54
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@Rogue We have yet to measure it directly, but every theory says that anti-Hydrogen should have indistinguishable spectral emissions and absorptions to ordinary Hydrogen (or for any other element/anti-element comparison, for that matter). –  Wedge Jul 12 '11 at 4:18
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Maybe much of the intergalactic dust in these transition regions has already annihilated, and the space between the matter/antimatter regions is getting larger due to the expansion of the universe. –  Carson Myers Jul 12 '11 at 6:01
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An interesting idea would be negative-mass antimatter, such that galaxies consisting out of antimatter would repel ordinary matter, drastically reducing the likelihood of such boundary annihilations... –  Tobias Kienzler Nov 4 '13 at 15:15
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