If black holes usually form when stars collapse, how do white holes form?
Nobody has ever observed a white hole.
A white hole is the time reverse of a black hole, and is thus forbidden due to the second law of thermodynamics.
To put it another way, we aren't going to be seeing any white holes out there for the exact same reason that we won't see scrambled eggs unfry and hop back in a shell.
Other universes? Seriously, there is no credible observational evidence of a white hole in this universe. That is the gold standard. Not everything that appears theoretically possible is known to physically exists in this universe.
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