An actual Dwarf galaxy in reality and in the fictional Stargate universe. The Pegasus Galaxy lies roughly half a million light years of galactic-north from the Milky Way. The Pegasus Galaxy is a companion of the Andromeda Galaxy and one of the 30-odd galaxies included in the Local Group.

See also Pegasus Dwarf

For all intentions of the Stargate universe, the Pegasus galaxy resides on a different galactic plane as the Milky Way. This supposition can be made due to the required 8th chevron calculation in the Stargate dialing sequence needed to reach the Pegasus galaxy.

Several million years ago, the Ancients, the builders of the stargate network, fled Earth to settle in the Pegasus Galaxy, in an effort to escape a devastating plague that was destroying their race.

For millions of years the Ancients (or Atlanteans, as they've been referred to) spread human life and prosperity throughout Pegasus, until they encountered a race of formidible aliens, the Wraith. The Wraith possesed technology and knowledge that rivaled that of the Ancients, and moved as a great scourge across the galaxy, feeding upon the life-force of the human populated worlds.

After a war that lasted several centuries, the Ancients were forced to retreat back to their great city of Atlantis, where they were besieged for many years, the powerful Wraith weaponry held back by the Atlantis defensive shield. Eventually all Ancients in the Pegasus galaxy were either killed or had Ascended, save for those within Atlantis, and the Ancients saw no reason to remain there. They shut down and submerged their city, and returned to Earth via the single Stargate in the Pegasus Galaxy capable of making the connection.

All of these events took place 10,000 years before the arrival of the Earth Atlantis team.