During the war with the Ancients, the Wraith captured many worlds in Pegasus with human populations to use as feeding grounds. They also control some uninhabited planets which they use as outposts, construction sites, and research bases.
In 2005, the Wraith's control over Pegasus was challenged after 10,000 years when humans from Earth rediscovered Atlantis and began to fight against them after accidentally awakening them into a galaxy where the humans numbers have yet to reach a level where the Wraith could feed causing a number of incidents. It was in this time that several planets formed an alliance with others creating a united government while the Wraith warred against each other. A number of the Wraith's all important and seat of government, known as Hive ships were destroyed drastically reducing their fighting capability until a Wraith Queen known as Death took control of the Wraith and began a drive to get her people to Earth.
In the aftermath of Death's defeat and subsequent death, the remaining Wraith forged a peace agreement and, as such, half of the Pegasus galaxy controlled by the Wraith was given to the Tau'ri. (SGA: "The Lost") (SGA: "Legacy")