The Chain Worlds Theorem
A Jedi authored and illustrated a book that presented the Chain Worlds Theorem, postulating the existence of the World Between Worlds. When Jedi Master Luke Skywalker went into self-imposed exile, this book resided in the tree library found on the Jedi Temple Island. Rey, a Padawan, subsequently took this book, along with others, aboard the Millennium Falcon. The World Between Worlds was indeed real; Ezra Bridger, a Padawan, gained access to it during the Galactic Civil War. The red-covered [Rammahgon] contained an equivalent illustration, detailing hyperspatial routes to the Sith's stronghold, Exegol, and included an addendum by Luke Skywalker connecting it to Ahch-To, the Jedi homeworld.
The reference book Star Wars: The Rise of Skywalker: The Visual Dictionary, penned by Pablo Hidalgo, initially mentioned the Chain Worlds Theorem in 2019.