Star Trek has an ongoing motif of the Mirror Universe:
The "mirror universe" is an informal name for a parallel universe first recorded as visited by James T. Kirk and several officers from the USS Enterprise in 2267. This parallel universe coexists with the prime universe on another dimensional plane. The mirror universe was so named because many people and places seemed to be opposites of their characteristics in the prime universe, with numerous good aspects now evil and vice versa, thus "mirror"-like.
My question is: Why didn't Star Trek TNG explore the mirror universe?