The Ashlan Expanse was a region of space in the Unknown Regions. It contained planets such as Kal'Shabbol and Had Abaddon. It was mostly unexplored but fairly important nonetheless. In 29 ABY it was invaded by the extra-galactic Yuuzhan Vong invaders. However, it was reclaimed and taken back.
Behind the scenes[]
The idea of an "expanse" came from the "Delphic Expanse", seen in the third season of Star Trek: Enterprise.