Warning: SPOILERS for Star Trek: Discovery Season 4, Episode 7 - "...But To Connect"
The existence of the Multiverse in Star Trek was confirmed by Dr. Ruon Tarka (Shawn Doyle) in Star Trek: Discovery season 4. Tarka was introduced in Star Trek: Discovery season 4, episode 5, "The Examples," as one of the United Federation of Planets' greatest scientists brought in to unlock the secrets of the Dark Matter Anomaly AKA the DMA. Tarka built a miniature model of the DMA that could have destroyed the USS Discovery, but the mad scientist's return in Star Trek: Discovery season 4, episode 7, "...But to Connect" revealed Tarka has greater knowledge of Star Trek's Multiverse.
Star Trek: The Original Series introduced the Mirror Universe early in season 2, and the parallel reality where the Federation and Starfleet were populated by evil counterparts remains one of the franchise's most intriguing concepts. However, Star Trek only used the Mirror Universe sparingly, with Star Trek: Deep Space Nine devoting a few episodes set in the alternate universe before Star Trek: Enterprise did a prequel two-parter predating the Mirror Universe's appearance in TOS. It wasn't until Star Trek: Discovery that the Mirror Universe was explored more thoroughly, thanks to Emperor Philippa Georgiou (Michelle Yeoh) becoming a key member of Discovery's cast. But Star Trek canonically existed in the Prime Universe along with the Mirror Universe as its dark offshoot until 2009, when J.J. Abrams' Star Trek movie introduced the alternate Kelvin Timeline. With that, Star Trek now had three canonical timelines, and Star Trek: Discovery season 3 even revealed a 24th-century time soldier named Yor made the fatal jump from the Kelvin to the Prime Timeline.
However, Dr. Ruon Tarka's revelation in Star Trek: Discovery season 4, episode 7 canonically establishes that Star Trek does, indeed, have a vast Multiverse beyond the Prime, Kelvin, and Mirror universes. Tarka confirmed the Multiverse to Book (David Ajala) when he told the Kwejian pilot that he needed to destroy the DMA so that he could turn home to an alternate universe. When Book assumed Tarka meant the Mirror Universe, the mad scientist objected and said, "You say that like it's the only one." Tarka went on to describe infinite parallel universes that each "exist with their own quantum signature." Although Ruon previously said he hails from Risa, Tarka described the "home" he intends to go to as the unspecified alternate universe where his unnamed partner hails from. Tarka met his partner when they were both prisoners of the Emerald Chain. However, Tarka believes his partner has already escaped to his home universe where Ruon intends to join him.
Star Trek has already teased the existence of the Multiverse in the novel Star Trek: Discovery: Dead Endless by David Galanter. Although Star Trek novels are usually not considered canon, the books and comics based on Star Trek's Paramount+ series are often penned by writers involved with the TV show productions and details can be incorporated into the Star Trek shows. Dead Endless depicted the USS Discovery encountering a "Multiversal Federation" within the mycelial network, which enables the Discovery's instantaneous travel via its spore displacement hub drive. But Dr. Tarka speaking of the Multiverse on an episode of Star Trek: Discovery officially makes it canon.
Star Trek now has the exciting potential to expand to infinite universes the way the Marvel Cinematic Universe has with What If...? and Spider-Man: No Way Home and the DC Universe has in the Arrowverse and the DC Extended Universe movies. At the very least, it's possible Star Trek: Discovery season 4 will give a glimpse of Star Trek's fourth universe if Dr. Tarka is able to successfully travel there. Confirming the Multiverse seems to be a tacit promise that Star Trek could begin to explore more parallel timelines, especially since Dr. Tarka has created a next generation spore drive that may be able to accomplish Multiversal jumps. The Multiverse could possibly even be the next frontier Star Trek: Discovery will boldly go to in season 5 and beyond.
Star Trek: Discovery season 4 returns February 10, 2022, on Paramount+.