The BTS Universe, also known as the Bangtan Universe or BU, is an alternate universe created by Big Hit Entertainment that winds through the group's output. The webtoon Save Me closely follows the story line, along with their book HYYH: The Notes 1. The chronology of the universe began with the song "I Need U" and spans to the present time. It tells the story of the seven members in an alternate reality and depicts their anxieties and uncertainties as they confront their future. In a corporate briefing held on February 4, 2020, Big Hit also announced the upcoming release of its sequel HYYH: The Notes 2, the recreation of five BTS songs into five picture books, and an upcoming drama to be produced in collaboration with Chorokbaem Media and renowned screenwriter Kim Soo Jin. Speaking to Billboard, Michelle Cho, an assistant professor of East Asian Popular Cultures at the University of Toronto, said "As an audio-visual experience, "I Need U" inaugurated the Bangtan Universe—the coming-of-age storyline that brilliantly integrates the Most Beautiful Moment in Life [album] trilogy and continues in music videos, "concept videos" (mini-films), and the multiform, open-ended narrative that's been serialized in album liner notes and Twitter and Instagram posts."
Mariejo Ramos from The Inquirer said about the universe "no other artists have successfully mixed the same literary technique to pop music in such a scale." They used the alternate universe and literary books, such as The Ones Who Walk Away from Omelas, as a framing device for their albums. Mixing the two has created an interactive environment for their fans who make theories when new material or hints are released.