The Gamma Quadrant isn’t empty, it isn’t just a bunch of planets. It’s bound together by the Dominion, a very very tough, very smart, very old civilization, run by the mysterious Founders, who are experts in genetic engineering, and who turn out to be Odo’s people, the Shapeshifters. They then go and engineer these slave races that do their bidding. Essentially, the two main slave races were the ‘carrot’ and the ‘stick’. The carrot being the Vorta, who would come to your planet and say “Hey, you’re nice people, here’s some M-16s and some popcorn, and whatever else you want baby, alcohol, fire-water? All you have to do is sign this little contract and we’ll make you cool.” Then there’s the Jem’Hadar. So the Vorta say “Oh, you don’t want to play ball? Then meet these guys. They’re gonna kick your ass.”