Pegasus Galaxy (Stargate)
The Pegasus Galaxy is a galaxy that features in Stargate.
The Pegasus galaxy was meant to be a safe haven for the Ancients fleeing the galaxy wide plague which was circulating amongst their population. According to the show, the Ancients inhabiting Earth fled in a City Ship Atlantis and later established a colony in the Pegasus galaxy. This was done in the hopes of rebuilding the Ancient civilization and restoring it to its former glory. The plague itself is thought to be of Ori design due to its similarities with the disease inflicted by Ori Priors to subjugate worlds. The Ori is therefore held responsible for abrupt dissapperance of the Ancients from this plane of existence with hundreds resorting to ascension to escape the plague while those who did not were inevitably dead.
For millions of years the Ancients, or "the Ancestors" as they became known in the Pegasus Galaxy, spread human life and prosperity throughout Pegasus, just like they had previously done in the Milky Way. They also established a vast network of Stargates on over a thousand planets, similar to the ones in the Milky Way.  However, the two gate systems were incompatible, and thus unable to dial between each other without a specific control crystal interfaced with the DHD. This was apparently done by the Ancients deliberately to try and isolate the Wraith in Pegasus and prevent them from spreading into other galaxies.  For some time, their empire flourished, until they stumbled upon the Iratus bug. These mutated from contact with humans, to form Wraith. The Wraith moved as a great scourge across the galaxy, feeding upon the life-force of the human populated worlds. The Ancients tried to fight the scourge (including creating the first Asurans as an attempt to oppose the Wraith) and although the Ancients' technology was significantly more advanced, the sheer numbers of the Wraith forced the Ancients into submission. 
After a war that lasted several centuries, the Ancients had lost most of their empire to the Wraith, and were forced to retreat back to their great city of Atlantis. There, they remained besieged for many years, submerged in the ocean to protect the city from the Wraith.  At that point, all other Ancients in the Pegasus Galaxy had already been killed or had ascended.  Eventually the Ancients in Atlantis realised the war with the Wraith had been lost, and returned to Earth via the single Stargate in the Pegasus Galaxy capable of making the connection.  However, they had hopes that their descendants would eventually discover Atlantis.
For the next 10,000 years the Wraith plundered the Pegasus Galaxy, feeding off the humans originally seeded by the Ancients, and periodically taking intervals for hibernation. Many planets were totally wiped out, while others were used like breeding pens, being periodically revisited by the Wraith, on feeding expeditions (cullings).
After Daniel Jackson found the gate address to the lost city of the Ancients at the Ancient outpost in Antarctica, on Earth, the Tau'ri sent an expedition to the Pegasus Galaxy, where they discovered the city of Atlantis in near perfect condition. Unable to return to their own Galaxy, the expedition was forced to take up residence in the city. However, during an attempt to discover new ZPM's to replace the depleted ones on Atlantis, the expedition came in contact with the Wraith, accidentally interrupting their hibernation cycle prematurely.
Hive by hive began to awaken, again feeding on the human worlds, without sufficient "live stock" to sustain the entire species. During the first encounter between the Wraith and the Atlantis expedition, the Wraith had learned about the existence of Earth and the Milky Way, a rich new feeding ground with countless billions of humans. However, without Intergalactic hyperdrive technology as the Ancients, Asgard, Tau'ri and other advanced races possessed and the Pegasus Stargate network unable to dial outside their Galaxy, the Wraith were unable to deduce the exact location or any way to get there...leaving to a situation with the entire Wraith species awake and too little food to sustain them. 
With the galaxy's inadequate amount of humans to sustain their entire population, Wraith factions began to form to gain access to the sparse feeding grounds.  Eventually, the factions began to fight each other over the inadequate food supply, with stronger groups attacking the weak. Soon, the conflict erupted into a full-scale civil war, offering hope to the humans living in the Pegasus Galaxy.  Later on, however, after the Atlantis city-ship left Lantea after a Replicator attack via a stargate satellite, the city soon came to drift through the galaxy after the hyperdrive shut down too soon. There is known to be one large asteroid belt in the Pegasus galaxy after the city had to skirt its edge.
During a heist on the replicator home world to steal a ZPM so that Atlantis can be powered back up and flown to a new planet, the Atlantis team discover an inactive protocol in the replicators base code that would force the replicators to attack the wraith. With Atlantis safely on the surface of its new home and the replicator inactive protocol now active the cities long range sensors detect a massive fleet of replicator war ships heading for a wraith planet, bringing hope to the humans living in the Pegasus galaxy.
In season four, it is discovered that the Wraith themselves were responsible for deactivating the protocol causing the Asurans (replicators) to attack them. As the war between Wraith and Asurans continued, the Asurans began to attack human worlds in order to cut off the Wraith's food supply. The Wraith and the Atlantis expedition were forced to work together to modify the original virus that deactivated the attack protocol.
- Ronan Dex :
- In reality, there are two galaxies in the Local Group called Pegasus Dwarf; the Pegasus Dwarf Spheroidal Galaxy and the Pegasus Dwarf Irregular Galaxy. It has not been explicitly stated which of these is the galaxy in the Stargate universe. However, in this discussion regarding the new McKay-Carter Intergalactic Gate Bridge, General Hank Landry states that the distance between the Pegasus and Milky Way galaxies is "three million light-years," suggesting that the series takes place in the Pegasus Dwarf Irregular Galaxy.
- Stargate: Atlantis: