The McAllister C-5 Nebula was a protostellar nebula in Sector 21527 of the Alpha Quadrant. This nebula, in Cardassian territory, was approximately seven light years from the Federation-Cardassian border and eleven light years from the Federation system Minos Korva. The particle flux inside the McAllister Nebula prevented starships from staying inside it 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 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")
This nebula's quadrant of origin is inferred based on the position of Minos Korva as seen in the star chart appearing in the Star Trek: Picard episode "Maps and Legends". Based on this chart, from 2399, the McAllister C-5 Nebula was in or near to Federation space.
In the Star Trek Online timeline, the USS Enterprise-E visited the nebula to conduct research in the year 2390. During their study, scans revealed quantum singularities which hinted at the presence of Species 8472 in the area.