Genesis is an artificial world created by Forerunner Builders to act as the source for "seed worlds", planets that were to be used as temporary homes for civilizations to keep them from dying out. In addition, Genesis is home to a gateway to the Domain, monitored by installation monitor 031 Exuberant Witness. When the Halo array was fired to rid the galaxy of the Flood, it also destroyed the gateway to the domain, though it was eventually repaired enough to be accessible again.
In 2557, after the New Phoenix Incident, Cortana was sent through a slipspace portal to Genesis, who then gained access to the Domain though the gateway. Using this access, Cortana took control of the Forerunner Guardians placed throughout the galaxy and used them to take control of much of the galaxy and beginning her faction of artificial minds known as the Created. Though both Spartan-II Blue Team and Spartan-IV Fireteam Osiris were sent to Genesis, neither team was able to stop Cortana, nearly getting themselves imprisoned along the way.