The Dominion were a powerful galactic empire from the Gamma Quadrant.
The Dominion was founded over 10,000 years ago by a race of changelings who later became known as the "Founders". Having been persecuted by "solids" for many years, they retreated to a rogue planet and decided to impose "order on a chaotic universe". They began enforcing their will on lesser species and elevated races such as the Vorta to sentience. Around 2,000 years ago, the Founders' genetic sciences evolved to the point where they could create entirely new species from scratch. From this, they created the Jem'Hadar, the most brutal soldiers in the known galaxy. (The Dominion War Sourcebook: The Fires of Armageddon)