Several Star Trek series visit the Mirror Universe at some point, but The Next Generation isn't one of them. That's not an accident, either.

However, TNG avoided delving into the Mirror Universe and following up what was seen in Star Trek: The Original Series ' "Mirror, Mirror." This wouldn't happen until Star Trek: Deep Space Nine.

Similarly, our universe differs from its mirror image. The weak force—the force that’s responsible for radioactive decay—is felt only by left-handed particles.