Richard Feynman had a satirical theory that any complex mathematical problem/theory when stated in layman's terms becomes self-evident. This is the perfect example of this.
There may be other universes created by whatever mechanism created ours. It is possible also that in some such mechanisms, indirect effects of those universes may be observable.
There's a limit to how much we can hypothesize about the nature of other universes, given we don't know if they actually exist. It'd be foolish to claim "QED" either way.
Since we can't possibly know anything about these universes for all intents and purposes we can say that they don't exist. This QED is more than acceptable.
There may be other universes created by whatever mechanism created ours. It is possible also that in some such mechanisms, indirect effects of those universes may be observable.