The Argolis Cluster was a stellar cluster consisting of six star systems that was located in the Alpha Quadrant. One of these systems included the inhabited planet Tagra IV, which was the homeworld of the Tagrans. (TNG: "I Borg", "True Q")
In 2368, the USS Enterprise-D conducted a survey of the Argolis Cluster's star systems as a possible prelude to Federation colonization. During the survey, it discovered a crashed Borg scout ship with one surviving drone, who would later be known as Hugh. A few months later, in 2369, the Enterprise returned to the Argolis Cluster to deliver relief supplies to the ecologically devastated Tagra IV. (TNG: "I Borg", "True Q")
During the Dominion War, the Dominion concealed a large sensor array at the periphery of the Argolis Cluster. The array allowed them to monitor allied ship movements over five sectors, providing them with a substantial tactical advantage. In 2374, the USS Defiant, commanded by Lieutenant Commander Jadzia Dax, successfully destroyed the array by moving through the cluster instead of around it. The Dominion forces were unprepared for an attack from that direction, as they believed any ship inside the cluster would be destroyed by the extreme Gravimetric distortion and gravimetric shear. (DS9: "Behind the Lines")
René Echevarria wrote three of the episodes that mentioned the Cluster.
According to Stellar Cartography: The Starfleet Reference Library ("The Dominion War: Strategy and Battles, 2373-75"), the Alliance raided the Argolis Array in April 2374.
According to the script for "Behind the Lines", this cluster's name was pronounced as "ahr-GO-liss".