What happens to surrounding particles when matter-antimatter annihilation occurs?

[Simon Bridge]

Um - as interesting as working out how many atoms there are in etc etc isn't it a bit off topic? I intended my statement to be an absurd understatement :) Even attempting to manufacture, without bothering to contain, the mere 1g that was my example would be silly.

 

[L-x]

Volume is not a meaningful quantity here - most of the matter is very thin interstellar plasma of variable density, and the remaining part is mainly plasma in stars. Just a small fraction is solid.

How to estimate the matter density? Mainly: observe it. The total amount of matter is then given by a multiplication with the size of the observable universe.

That's precisely the point I was trying to make.