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We know that antimatter exists. Could it be that galaxies near us are made of antimatter? How do we know that antimatter has vanished from space shortly after the Big Bang? Antimatter must have very similar propreties to matter. How can we distinguish it by looking at it? As far as I know, it doesn't emit antiphotons! Unless we go in other galaxies, take some samples and confirm that it is not antimatter, we cannot say antimatter has vanished!
Not likely, at least in our local super-cluster called the Virgo cluster, which contains about 2000 galaxies. If so, then we would see lots of radiation coming from matter-antimatter annihilation. In addition we would probably see lots of anti-particles in cosmic ray spectra. The spectra observed by many earth bound and orbiting detectors do not show any evidence for excess other than that produced by standard processes.
The first decent theory of how there could be a matter-antimatter asymmetry was proposed by the Russian theorist Andrei Sakharov in 1967. It's a bit complicated but seems to be in at least qualitative agreement with present day experiments. http://en.wikipedia.org/wiki/Baryogenesis
(published on 06/09/2010)
Follow-Up #1: why more matter than antimatter?
What made there be a different amount of antimatter compared to matter in the big bang.
- Ryan (age 13)
Ryan- I've changed the follow-up link to one closer to your own question.
We can elaborate a little on Lee's answer. Some parts of the answer are probably too fancy for you now, but I've included some vocabulary to help search for more info.
If everything started out with equal densities of matter and antimatter, then it looks like some later process caused some imbalance. That's not impossible. We can measure something called "CP violation". Those are processes in which antimatter behaves unlike matter. In events where the final state drastically differs from the initial state (strongly "T-violating") these physical effects can lead to the production of different amounts of matter and antimatter. The moments just after the big bang had several stages in which the universe changed drastically, events called "phase transitions". Andrei Sakharov proposed that such a phase transition drove the matter-antimatter imbalance via the CP-violating processes. So far, however, no specific version of this idea has been able to explain the extent of the imbalance. That's not too shocking because we don't know much yet about physics at the very high temperature scales involved.
(published on 06/06/11)
Follow-up on this answer.