mirror universe

noun2

1. •
   An alternative universe within a fictional work, separate from the universe normally depicted, where familiar characters and settings are present but in a systematically "reversed" manner, often by being evil or otherwise hostile.

   • Star Trek: The Original Series introduced a mirror universe in which humans had aggressively conquered parts of the galaxy rather than forming the peaceful Federation, and the setting was revisited in several of the franchise's spinoff shows.
2. •
   (rare) A hypothetical universe, a counterpart to this universe but with certain elementary particles reversed in a specific way.