The ruling caste of the Gamma Quadrant's Dominion are the Changelings that spawned Odo.
They intentionally kept their planet secret after a history of persecution by "Solids" that fueled their all-powerful drive to control the chaos around them. Beginning some 2,000 years ago, they sought to maintain "order" out of this chaos with a rule based on strict obedience, enforced by the Jem'Hadar troops they genetically bred into chemical addiction for control, and administered by the Vorta they cloned — both of whom worship the Founders as gods.
In natural form, the Founders exist as a gelatinous liquid and can unite in what is described as the Great Link — which they can leave and reform into any any shape whenever possible. Until Odo's torn loyalties led him to defend the U.S.S Defiant by accidentally killing a Changeling spy in 2371, no Founder had ever harmed another.