Henry wrote: << How do we know whether something we're looking at in a telescope is matter or anti-matter?

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Josh wrote: << I think the main answer is that space isn't empty, and if there were a boundary between matter and antimatter anywhere we would see all the gamma rays coming from the annihilations at the boundary.

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I would add that AFAIK we have no idea what fraction of the (spatial) universe we're able to see. Usually it seems people assume it's most of the universe, or at least that the rest of it doesn't vary greatly from what we see. But what we can see could be an unimaginably small fraction of the actual universe, which might even be infinite. So, what we see may well be vastly uncharacteristic of the universe at large. --Dan I sleep as fast as possible so I can get more rest in the same amount of time.