The M'Tar Domain was an area of subspace where the M'Tar lived.
Hahn Jun-Seok once collapsed a portal into the M'Tar Domain by exploding the USS Artemis. Before the portal closed, he sent a message to the USS Prospect, warning M'Tar forces were growing once again, being ready soon to launch another attack against the Milky Way Galaxy. (Star Trek: The Prospect Chronicles: "Lady Lazarus")