Symmetry breaking is an explanation, but we don't know if it's the right explanation. It's just the best we have at present.

That's what is explained in the Janus model, both universes coexist in the same space and are rejecting each other through gravity, matter "won" over antimatter only by a tiny bit but sufficient to allow large portions of matter to remain. Though the appearance of both at the early times of the universe was in beyond colossal amounts, both being annihilated by each other when they met.

edit: typos