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Chalnoth
#3
Nov19-09, 12:07 AM
Sci Advisor
P: 4,840
Quote Quote by AaronKnight View Post
Is it possible that a far off galaxies that we are unable to reach are made up of anti-matter? How do we know that they are made up of matter?
We're pretty darned sure this can't be the case. As mathman noted, if this were the case we would see a lot of matter/anti-matter annihilations. We have looked for these, by the way, and don't see them.

But perhaps even more compelling is the existence and smoothness of the cosmic microwave background: this stuff was emitted when our universe cooled from a plasma. At the time, our universe was uniform to one part in 100,000. It was almost perfectly smooth. Because of this, there just wasn't anywhere for the anti-matter to "hide" from the matter to avoid annihilation. So there really can't be much of any anti-matter left around.