mirror universe

Noun

mirror universe (plural mirror universes)

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.
   • A mirror universe of fermions and forces isomorphic to but distinct from those we see couples directly to our universe only by gravity. […] Consequent effects are contrary to experiment. The possible existence of such a mirror universe is thereby excluded.1986 January 30, S.L. Glashow, “Positronium versus the mirror universe”, in Physics Letters B, volume 167, number 1, page 35