Hi, I keep reading about that the universe is supposed to be composed of only matter. And that this matter was an excess amount compared to anti-matter after the big bang. So most of the matter and anti-matter annihilated each other and some matter was left over. My question is: What is the reason to believe that everything we can observe in the universe is composed of matter ? What is wrong with assuming that a far away galaxy is entirely composed of anti-matter (such that the amount of matter in the universe is equal to the amount of anti-matter achieving perfect symmetry in this respect) ? Anti-matter would appear to us the same way as matter and since it is not globally charged and therefore doesn't attract each other electro-magnetically (which would be the only difference between matter and anti-matter) I don't see any problem with this idea. This way the problem of baryon-asymmetry could be completely avoided. http://en.wikipedia.org/wiki/Baryon_asymmetry It turns out in this link there are some explaination and also the one I suggested here. So it seems the only reason not to believe it is that there are no seen sources of gamma-radiation where matter and antimatter come into contact. But on the other hand galaxies are drifting appart making it more and more unlikely for such event to appear. Also if they would touch (like ages ago lets say) they would create lot's of gamma radiation virtually pushing the 2 clusters of matter and antimatter appart, wouldn't they ? Any thoughts on this ?