After nearly 60 years of existence, many things about Star Trek: The Original Series are iconic from the (never actually said) line, "Beam me up, Scotty" to Leonard Nimoy's Vulcan nerve pinch. The Season 2 episode "Mirror, Mirror" is another, for its introduction of the Mirror Universe. The goatee Spock wears in the episode even started the pre-internet meme of "evil" versions of characters sporting distinct facial hair. In fact, Star Trek: Discovery is the show that spent the most time in the Mirror Universe, with seven episodes over two seasons. Deep Space Nine got to visit the Mirror Universe five times during its run and Enterprise -- the series that followed Star Trek: Voyager -- visited it for a two-parter.
Source：https://www.cbr.com/star-trek-voyager-mirror-universe/
