The Proteans were an ancient civilization who were believed to have been extinct by the 24th century. This race was noted to have held sway over most of the galaxy about a billion years before any of the species within the United Federation of Planets had learned to harness fire. Their existence became known to Starfleet in 2367 which led to the race being given the code name "Protean".
By 2375, Starfleet had established the Pandora facility to study the ruins of a Protean world though it was suspected that even if they worked for decades they would not crack all of the secrets of the ancient race. However, primary research began on an artifact known as the Codex. At some point in the year, the chief cryptologist Axael Krinn took the people within the facility hostage, which prompted the USS Anansi to take Ezri Dax to the base in order to facilitate in the rescue of the captives, Jadzia Dax having been a member of the original team. Ezri was successful in stopping Krinn, activating the Codex and the Catalyst, which was believed would "restart" entropy once the universe ended and create new life. (DS9 - Prophecy and Change short story: "Chiaroscuro")