It’s known from The Mandalorian that Moff Gideon got his hands on the Darksaber, but its never clearly stated how that happened. However, it can be assumed that it happened during the Great Purge that devastated the people of Mandalore. In The Mandalorian season 2, episode 3, Bo-Katan and her two Mandalorian companions launch an attack on an Empire ship in an effort to retrieve the Darksaber from their enemies. During a confrontation with an Imperial captain (Titus Welliver), Bo-Katan demands to know if “he” has the Darksaber, in an obvious reference to Moff Gideon. The captain’s answer is “if you’re asking, you already know”. Bo-Katan’s lack of certainty about whether or not Moff Gideon has it suggests that he didn’t steal it from her directly.
If Moff Gideon battled Bo-Katan in the Great Purge and took the Darksaber, she wouldn’t ask if he has the weapon. Bo-Katan likely lost track of it when Gideon and the Empire attacked Mandalore. When her forces were taking heavy losses, she could have been severely injured in the fight with Gideon’s men. Following the battle, she could have discovered that the Darksaber was taken in the heat of combat. If so, she could have assumed he stole it but wouldn’t have been completely sure.
After the new episode, Bo-Katan’s doubts have been washed away, and the identity of the Darksaber’s owner is clear. As season 2 progresses, her quest to take back the Mandalorian artifact and defeat Moff Gideon will continue. With just two Mandalorians on her side, retrieving it from a villain as capable as him is sure to be a challenging task, especially since Mando has opted not to help her. That being said, whatever happens when he finds Ahsoka Tano (Rosario Dawson) could lead him back into a conflict with Moff Gideon. By the time The Mandalorian season 2 ends, the saber could find its way back to Bo-Katan, or it could be that it will be passed to a different Mandalorian.
More: Who Is “The Heiress”? Mandalorian Season 2, Episode 3 Title Explained