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Anti-matter galaxies

Hi all-
Leigh offers:
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A great leap to fall flat on your face. If a galaxy is all antimatter
there would be no annihilation radiation at all coming from it. matter-
antimatter inhomogeneity should only be seen on the very largest scales
in the hierarchy of the universe if it exists at all.

To my knowledge few astronomers have spent much time looking for 511 keV
gamma rays coming from galaxies; one would not expect to find it except
in one place - the center of a galaxy where positrons can be created in
the energetic processes at the core. 511 keV gammas have been seen from
the center of our own galaxy, of course. The source(s) is variable.
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The annhilation radiation would come from the collisions of
matter galaxies with anti-matter galaxies.
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Leigh further propounds:
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The .51 MeV
gamma-ray pairs are probably the "pure energy" that is occasionally
referred to in popular writings.

That's probably true, but in that case corn flakes are "pure energy" too.
Why do popularizers have to confuse the public by introducing gratuitous
simplifications? Is "pure energy" somehow more concrete than "gamma rays"?
Given that the latter exist and the former does not, do we really have to
continue to do that?
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So how do we decide whether the popularizers are wrong or some
physics teachers are being pompously pedantic?
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Also, Leigh mistakenly asserts:
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One expects that there are subtle differences between atomic
spectra and anti-atomic spectra. These differences arise because
certain atomic transitions show the results of interference between
electromagnetic and weak interactions. Because the weak interactions
are not invariant under the CP transformations, the interference
effects can look different in the anti-atomic spectra.

While these have been conjected they have not yet been seen in the
laboratory. What I said is that I doubt they will ever be seen in
astronomical observation even if antimatter is present.

Leigh
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See, e.g., the measurement of E1-M1 mixing by Meekhof, et al.,
PRL 71(1993)3442.
Incidentally, I should have said "C and P transformations"
rather than "CP transformations".
Regards,
Jack
"I scored the next great triumph for science myself,
to wit, how the milk gets into the cow. Both of us
had marveled over that mystery a long time. We had
followed the cows around for years - that is, in the
daytime - but had never caught them drinking fluid of
that color."
Mark Twain, Extract from Eve's
Autobiography