It's just semantics. The Milky Way galaxy used to be the 'universe' - until what were thought to be local nebulae were found to be distant galaxies - then for a while, they were called 'island universes'. We're at the point where several different kinds of 'multiverse' (collections of causally isolated spacetimes) are predicted by current theories, so it's just a question of deciding the nomenclature.You can't be "A" and "-A" at the same time, we must either change the definition of Universe (from all that exists, to something else) or recognize there can't be multiple of them.
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