The Dallos Cluster was a massive purple blue hued gaseous cloud which borders nearly one fourth of the Oralian sector. (Star Trek: Pioneer (STP); Star Trek: New Horizons)

Once Tyson Calok was traced there by the USS Pioneer, only to have them witness him destroying three Borg cubes. During the Coalition War, Breen and Starfleet forces have battled within it. ("In Custody")
Breen dallos

A Breen ship near the Dallos Cluster Star Trek: Pioneer.

In early 2383, the Breen made a third advance into the Dallos Cluster and ran into the USS Horizon, causing the Federation starship to risk an early jump, which lead the the ship's disappearance. ("Jump")