The Fifth Galaxy was represented at the Galactic Council by Zephon, its master, in 4000. (DW: The Daleks' Master Plan) Prior to this, in 3982, it seems the Embodiment of Gris ruled there until Zephon disposed it. (BFA: Neverland)
:Zephon's presence on the Council, made up of representatives of the Outer Galaxies seems to imply that the Fifth Galaxy may belong to this group. The Fifth Galaxy may or may not have a relationship with the terrorist group Galaxy 5, active just prior to 4000.