firstly, how do we know this? and secondly, would it not be possible that either antimatter and matter were produced in separate regions or randomly and if randomly it makes sense that in any given region only one or the other would predominate?

  • $\begingroup$ Possible duplicates:physics.stackexchange.com/q/26397/2451 and links therein. $\endgroup$
    – Qmechanic
    Nov 20 '19 at 14:50
  • $\begingroup$ There are two separate questions here. Please ask O E question per submission. $\endgroup$
    – my2cts
    Nov 20 '19 at 17:55

If there were different regions, some with matter and others with antimatter, we would see the telltale signs of annihilation at the boundaries between them, there is none.

As to why there's only matter when both matter and antimatter should be produced in equal amounts, don't know, pretty big mystery.

  • $\begingroup$ could there not be empty space between the different regions and if there had been annihilation at the boundaries in the past, they no longer occur as the annihilation caused the amount of empty space to increase or even drove the regions farther apart? $\endgroup$
    – releseabe
    Nov 22 '19 at 13:43
  • $\begingroup$ If there was annihilation in the past we could still see it happening as far away ancient photons are reaching us now. As for empty space between them, I don't think so, even "empty" intergalactic space has about an atom per square meter, and there are a lot of square meters for annihilation to happen in. $\endgroup$ Nov 22 '19 at 15:08

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