The Nekrit Expanse was a huge, sparsely populated, nebular region in the Delta Quadrant containing an unknown form of interstellar gas. It was several thousand light years across and was impossible to chart due to its constantly changing structure. Because of its vast size, the Expanse functioned as a natural barrier of sorts, limiting contact between species on either side of the cloud. This accounted for the limited exposure of some species on the side of the cloud farther from the Alpha Quadrant, such as the Kazon and Vidiians to many of the technologically superior, and often more aggressive, species on the side nearer the Alpha Quadrant such as the Borg, Voth, and Krenim.
During the late 24th century there was a trade station at its periphery, overseen by Bahrat. The USS Voyager briefly stopped at the station in 2373 for provisions before beginning its month-long journey across the expanse. It was during this layover that Neelix revealed to Captain Kathryn Janeway that the Nekrit Expanse marked the limit of his knowledge of space and, he thought, his usefulness as a guide. (VOY: "Fair Trade")
First Officer Chakotay and Ensign Marie Kaplan performed a scouting mission of the Nekrit Expanse aboard a class 2 shuttle to find a faster route through it. Their mission proved difficult as the nebula completely scrambled the shuttle's navigational readings, making it nearly impossible to get a fix on their position. (VOY: "Unity")
Inside the Nekrit Expanse, they discovered an asteroid field at coordinates 113 mark 7. They also discovered a planet that contained a settlement of approximately 80,000 humanoid lifeforms on the western continent using rudimentary structures and technology.
Meanwhile, while crossing the Expanse, Tom Paris suggested renaming it the "negative expanse", because Voyager hadn't run across anything interesting for days. Moments later Voyager discovered a deactivated Borg cube. (VOY: "Unity")