The distant quarter of the galaxy reached through the Bajoran wormhole and home to the Dominion.
The Gamma Quadrant is one of the four quadrants of the galaxy, lying on the far side of the Bajoran wormhole from the Alpha Quadrant. Before the wormhole was discovered it was almost entirely unexplored, and the region proved to be home to a wide range of species and civilisations.
Its largest and most important power is the Dominion, which rules a large part of the quadrant through the Founders, Vorta and Jem'Hadar. The quadrant also contains many smaller peoples, some of them conquered by the Dominion and others independent, and a variety of curious worlds visited by the crew of Deep Space Nine.
Captain Sisko and his crew make the first explorations of the Gamma Quadrant in the early seasons of Deep Space Nine, using the Defiant and runabouts to explore. They meet the Karemma, the Wadi, and the Founders, and learn of the Dominion. These encounters lead to the Dominion's decision to invade the Alpha Quadrant, and the Dominion War follows.