From The Stargate Omnipedia
Milky Way Galaxy
Galaxy 80 to 100 thousand light years in diameter, consisting of 200 to 400 billion stars. The Milky Way is home to Earth
and its transplanted humans
, as well as dominant races such as the Jaffa
and (at one point) the Goa'uld
The galaxy is tied together by a sophisticated network of Stargates, which have the potential to number to 63 billion. These Stargates were originally positioned by the Alterans, who would later be known as the Ancients. Not only did the Ancients bring technology, but also their genetic potential, seeding the Tau'ri with their code of life.
Despite the extensive historical analysis performed off-world by Earth's Stargate Command, much of the Milky Way's history remains fragmented. Various Ancient texts suggest that a plague swept through the Milky Way anywhere from three to 50 million years ago. Following this event a super-weapon on Dakara was used to recreate all life in the galaxy. It is possible that the last of the Ancients departed our galaxy for another at this stage. Whether the vengeful Ori (the brothers of the Ancients) are responsible for this assault remains a mystery.
Following Ra's pillage of Earth, the Goa'uld were the dominant race in the Milky Way. Power-hungry Goa'uld forced the downfall of their race by devouring the creatures in elaborate ceremonies, leaving the symbiotes with zero population growth. The destruction of Goa'uld queens has contributed to this as well.
There is also evidence to suggest that the creators of the first Replicators once had significant ties to the Milky Way.
A number of ascended Ancients occupy a higher plane of existence relative to that of the Milky Way in order to protect the galaxy from the Ori, who occupy ascended space in their galaxy. It is also possible that these Ancients remain in Milky Way space simply to avoid their own conflict with the Ori. This fact is interesting, because it proves that ascended beings do not occupy all space on their higher plane, but only space in proximity to their interests. The Ancients have, until recent history, prevented all knowledge of the populations in the Milky Way from reaching the Ori.
The Milky Way is one of the two largest galaxies in the Local Group, a cluster of approximately 30 galaxies which share 10 million light years with one another. The other dominant galaxy is Andromeda, and includes the satellite galaxy of Pegasus. Pegasus, which was colonized by the Ancients after the Milky Way, is the only other galaxy thus far confirmed to contain a Stargate network. (Other galaxies with potential networks include the Ori galaxy, as well as galaxies occupied by the Asgard, such as Ida and Othalla.)
Fair Game - Thor admits that the Asgard would be unable to uphold the Protected Planets Treaty in the Milky Way if the Goa'uld called their bluff and engaged in a conquering spree.
Nemesis - After years of conquering the galaxy, Replicators return to the Milky Way.
Reckoning, Part 2 - The Dakara weapon, once used to form life in the Milky Way, is used against the Replicators.
Letters From Pegasus - Atlantis transmits a data burst back into the Milky Way to let Stargate Command know that they are, for the moment, alive.
Threads - Anubis is prevented from using the Dakara weapon to remake the Milky Way for his own purposes.
The Siege, Part 3 - The Daedalus arrives at Atlantis from the Milky Way for the first time.
Origin - The Ori discover a galaxy the Ancients were hiding from them -- the Milky Way, fresh for the picking.
Critical Mass - Colonel Caldwell is ordered to park the Daedalus in the void between the Milky Way and Pegasus galaxies in order to maintain an open communications link between Atlantis and S.G.C.
Allies - Eager for a fresh food source, a Wraith hive ship betrays Atlantis for the Milky Way's coordinates.
Camelot - After a year of threats, the forces of the Ori invade the Milky Way through the Supergate, bent on converting civilizations to worship the Ori.