The Dark Empire endnotes were miniature essays by Tom Veitch exploring the backstory of Dark Empire, as well as concept art by Cam Kennedy. They appeared in the back of each issue when they were first published, and were compiled at the back of the first two printings of the trade paperback. From the third printing on, the endnotes were dropped, thus making them extremely rare.
Among the topics covered are the history of the galaxy since the Battle of Endor, Palpatine's research into the dark side of the Force, superweapons, Nar Shaddaa, Byss, the Skywalker family, lightsabers and holocrons, and ancient Jedi. It also is the first mention of Ulic Qel-Droma, the Krath, and the events that would become the Tales of the Jedi series. However, as the background given in the endnotes was essentially a rough draft, it is very different from the actual Tales of the Jedi comics.
Much of the information in the endnotes was incorporated into the Dark Empire Sourcebook.