The Agni Cluster was a star cluster, a group of G class stars located in Federation space near Ferengi territory, somewhere in the galaxy's Alpha or Beta Quadrants.
History and specifics
In the year 2383, the Federation starship USS Enterprise-E conducted a survey of the Agni Cluster to locate class M planets suitable to create new colonies for the remaining refugees of the Borg Invasion of 2381. While there, they discovered the starship
, which had gone missing in the 22nd century. (TNG novel: Indistinguishable from Magic)