The McAllister C-5 Nebula was a protostellar nebula located inside Cardassian space in sector 21527, approximately seven light years from the Federation border and eleven light years from the Federation system Minos Korva.
The particle flux inside the McAllister Nebula prevents starships from staying in the nebula for more than 72 hours.
In 2369 the McAllister Nebula was used by Cardassian forces to hide their ships in order to begin a sneak attack on the nearby Federation system Minos Korva. However, this plan was blighted by Captain Edward Jellico, who commanded the USS Enterprise-D on a temporary assignment and detected the Cardassian ships inside the nebula. (TNG: "Chain of Command, Part II")