It was several thousand light years in width, making it impractical to rout around it. However, it was impossible to chart due to its constantly changing structure, and plasma storms, electrodynamic turbulence posed additional navigational problems and threats. The nebula scrambled navigational readings. Electrokinetic storms were not unheard of. (VOY: "Fair Trade", "Unity")
Because of this 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, Hirogen and Krenim. (citation needed • edit)
In 2373, Voyager encountered the Expanse, and decided to cross it. The USS Voyager briefly stopped at the station for provisions, and to attempt to find maps. 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")
The journey was expected to last a month, although First Officer Chakotay and Ensign Marie Kaplan performed a scouting mission aboard a class 2 shuttle trying to 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, Chakotay's mission discovered an asteroid field (twice) 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")