It was several thousand light years in width, making it impractical to route 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")
In 2373, USS Voyager encountered the Expanse, and decided to cross it. The 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")
In "Fair Trade", Neelix called the Expanse a territory, with a border.
André Bormanis tried to model the expanse on actual hostile regions within the galaxy that telescopes had identified. He cited supernova remnants as one such type of region.("Real Science with André Bormanis", VOY Season 3 DVD special feature)
In Star Trek Online the Dyson sphere from TNG: "Relics", a construct of the solanogen-based lifeforms (an Iconian servitor race), is teleported to the Expanse in 2409. In the Delta Rising expansion it serves as home base for the player character, part of a multinational task force operating in the Delta Quadrant.