And don't just put them on the moon, either. We're just as fucked if you split the moon in two as if you split the earth in two. Orbit it arround Venus or something.
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How do they know that other solar systems or other galaxies aren't antimatter? From what I understand, anti-matter shouldn't behave in anyway different that we can observe from such distances...though I suppose electromagnetics might be reversed; still not sure how you'd observe that without setting it up, though...