What's the spectrum of Antihydrogen?
Funny I typed this last night but maybe it was in positrons and annihilated itself. Anyhoo - here goes - again.
How do we know that distant galaxies aren't antimatter. Maybe "where the hell is it" - is all around us?
I still haven't had a decent explanation of why this can't be the case.
We are quite close to being able to measure the spectrum of antihydrogen, although it should be the same as normal hydrogen, if there do turn out to be some subtle differences then we will know what to look for out there.