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Experimental observation of matter/antimatter in the universe

I've heard a bit about the antimatter, matter inbalance. But I don't understand how it has been decided highly unlikely/impossible that some areas of the universe are made of matter, and others from antimatter. not necessarily specific galaxies, but maybe whole clusters/super-clusters?

Also I've also read that if this were the case then when opposite galaxies collide, it would create huge amounts of energy, which we would see, and as we don't see everything, all galaxies must be made of matter.

However when normal galaxies collide, the stars themselves do not collide with each other, so why when matter stars collide with antimatter stars would it be any different? Am I missing something like, antimatter is repelled by gravity or something?

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That question is about neighbouring galaxies being different? –  Jonathan. Nov 27 '11 at 19:49
    
But thanks for the astronomy question, it helps a lot, –  Jonathan. Nov 27 '11 at 19:51
    
I was looking at "How can astronomy evaluate if a far galaxy is made of matter or antimatter" when I decided it was close enough for a close. Do you feel I made a mistake? We could ask David or mbq to have a look. –  dmckee Nov 27 '11 at 20:22
    
Actually after reading the astronomy question it makes more sense, the only thing I don't understand is the space between the galaxies and clusters –  Jonathan. Nov 27 '11 at 20:30
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marked as duplicate by dmckee Nov 27 '11 at 19:43

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Even out in the edges of galaxies there is a significant amount of gas and dust - although stars don't collide when galaxies merge this smaller stuff does.

We don't see any background of x-rays from regions where stuff and anti-stuff are annihilating

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I'm assuming that most galaxies's dust has quite a low density, so the amounts of energy would be quite low, much much lower than if a star and "anti-star" collided! And if the portions were large enough, it would reduce the number of collisions, as there would be less 'edges'. Also any of the few collisions that happen would be significantly far away that we can't detect the gas/dust annihilating with the "anti-gas/dust" –  Jonathan. Nov 27 '11 at 19:07
    
Jonathan, Assumptions are ten a penny. –  Georg Nov 27 '11 at 19:18
    
Well less of an assumption, but space is a vacuum so the particles will try to spread out from high density into low density areas, and It's density must be not be anywhere near that of stars, else it form a star itself? –  Jonathan. Nov 27 '11 at 19:28
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A dust-dust bang is smaller than a star-star but there is an awful lot of dust and gas in a galaxy. –  Martin Beckett Nov 27 '11 at 19:30
    
And Wikipedia seems to say this as well, en.wikipedia.org/wiki/Interstellar_medium –  Jonathan. Nov 27 '11 at 19:31
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