I think the crux of the article is that matter-matter intereactions will act with positive gravity, antimatter antimatter interactions would act with positive gravity, but that matter-antimatter interactions would have negative gravity (or anti-gravity), while all having positive mass.
Antimatter wouldnt repel light though, since light would react with anti-matter the same as with normal matter, since it has no rest mass.
Also, if you found entire galaxies made completely of antimatter, how would you know? I remember reading something related to this in a popular science book, of how, if you were in communication with an alien race, how you could work out if they were made of antimatter, before you met them. Unfortunately, i cant remember the discussion, nor which book it is, but I shall try and chase it down.
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