After fleeing the Alteran Home Galaxy, the Ancients established quite an impressive empire in the Milky Way, based on their new homeworld, Terra Atlantus (Earth) . For some thirty million years, the Ancients resided in the Milky Way, continuing their expansion and placing Stargates on planets they visited in several different galaxies, regardless of whether they colonized it or not.
However, at some point, the Ancients began to suffer from a plague. The Ancients attempted to cure the virus, but were unable to. They began further research on Ascension as a way of escape, but only a few managed to escape that way. Unable to cure it, some of the surviving Ancients fled in the city-ship Atlantis and possibly other ships to the Pegasus galaxy.
Upon arriving in the Pegasus galaxy, the Ancients settled on Lantea, and renamed themselves the Lanteans after their new homeworld. After exploring the Pegasus galaxy, the Ancients found the galaxy devoid of sentient life. Seeking to fix the situation, the Ancient decided to seed humans on the various worlds. The Ancient Domain established some relations with the Humans of the Pegasus galaxy, such as the Dorandans, but for the most part the Ancients appear to have remained outside the seeded Humans developments, leaving them to their own devices.
However, one of the planets seeded was home to a creature known as the Iratus bug, and the Iratus bug fed on the Humans, becoming the Wraith. The Wraith destroyed the Ancient Domain until Lantea and Atlantis were virtually all that remained. Unable to win the war, the Ancients sank Atlantis under the ocean and returned to Earth. Although they did build a few outposts and construct ships after their return to the Milky Way, they did not rebuild their domain to anywhere near the size and power it once was. (SG1: "The Fifth Race", "Window of Opportunity", "Frozen", "Full Circle", "Lost City, Part 2", "Avalon, Part 2", "The Fourth Horseman, Part 1", "The Fourth Horseman, Part 2") (SGA: "Rising", "Before I Sleep", "The Siege, Part 2", "Trinity")