While there are several unanswered questions in Jujutsu Kaisen, this one is the most important.
Jujutsu Kaisen's manga reached its bittersweet conclusion in September 2024. The story begins with Sukuna's incarnation and ends with his defeat, completing one cycle of the never-ending battle. With Sukuna's defeat and the Jujutsu world starting anew, it's up to the new generation of sorcerers to set things right. Despite the seemingly rushed ending, the conclusion wasn't all bad. Nobara returns during the final fight against Sukuna and Megumi is safe. Yuji carries on Gojo's legacy as he remembers his final words. Even Sukuna, the proud and conceited King of Curses in Jujutsu Kaisen, accepts his fate and wishes to do things differently if he had a second chance.
While the ending leaves us with several unanswered questions, Yuji's Domain Expansion has to be the most important one. We know about the Domain Expansions of several sorcerers but not the main character. Yuji uses it against the main villain in the final fight and yet, we don't see how it works. In Jujutsu Kaisen Chapter 265, the manga features different scenarios where Yuji tells Sukuna about his childhood. However, there's still no clarification about it, even after the ending. All we know is that he gained it after awakening the Shrine technique. But is it a complete domain or an incomplete one? How does it work? What's its name? We will never find out the answers to these questions.
Domain Expansion is the pinnacle of Jujutsu sorcery. Only those with exceptional talent can hone their skills and master this ultimate technique. Even powerful sorcerers like Suguru Geto, Hajime Kashimo, Aoi Todo, and Kento Nanami never achieved this level. However, Yuji gains this ability during training before the fight between Gojo and Sukuna. Since swallowing the King of Curses' first finger, fans have witnessed Yuji's growth one step at a time. The progress is slow, but he perseveres as he constantly questions his purpose and the value of his life.
But when it comes to watching him use the best technique in the series, we don't get a satisfactory answer. A Domain Expansion isn't merely a power-up; it can often reflect a sorcerer's identity, beliefs, and inner world. For example, Sukuna's Malevolent Shrine reflects his godlike arrogance. The shrine echoes his self-image as a deity as he is destructive and death-bringing, as depicted by the skulls hanging on the shrine.
Throughout the series, Yuji struggles with his sense of identity, so giving him a proper domain could be a great conclusion to his character growth. Gege Akutami may have intentionally chosen to leave it out to focus on other aspects of the story. However, as the main character, not explaining his Domain Expansion makes his conclusion seem incomplete. But we will never find out the answer unless Gege plans a Jujutsu Kaisen sequel, which isn't likely as of now.