B'omar space was a region of the Delta Quadrant controlled by the suspicious B'omar Sovereignty, containing the moon where the Hansen family's ship, the USS Raven, lay wrecked.
The region appears in the fourth-season episode The Raven. Voyager asks permission to cross it to save months of travel, and the B'omar negotiators, led by Gauman, respond with an absurdly winding flight plan, strict speed limits and a ban on scans.
Talks collapse when Seven of Nine, suffering flashbacks of a black bird and Borg pursuit, takes a shuttle and breaks through the border. The B'omar treat her as a Borg threat and mobilise their fleet.
Tuvok follows her to a moon where a dormant homing beacon has been calling her. There she finds the Raven, her parents' vessel, and remembers her assimilation as a child. Voyager retrieves them under fire and goes round B'omar territory.