The Unknown race is an unknown species in the Milky Way galaxy. They appear highly advanced technologically, having ships with high yield energy weapons. They can understand or at least translate English.
They made contact with the Tau'ri in 2004 when they attacked the Prometheus and captured its crew. Though these aliens were never seen, Major Samantha Carter made a deal with them. She helped them get their ship out of a Interstellar cloud and in return, they returned the Prometheus' crew, before peacefully departing. (SG1: "Grace")
Five years later the aliens appeared in the Pegasus galaxy, once again testing the Tau’ri. They engaged the George Hammond commanded by Colonel Carter. They repeated their actions during their first encounter and the colonel was faced with illusory figures of Rodney and Sheppard. Once she realized that the aliens were testing her reactions only then they decide to directly communicate with her.(SGA: "Pleasure Cruise")
The aliens claimed to be an advanced race that by, their own admission, predating the Alliance of Four Great Races who they say at its formation were just beginning to scratch the surface of the universe. While not a member of the Alliance of Four Great Races, they were in correspondence.
They have been preoccupied with their own affairs in their own galaxy. When they returned to the intergalactic scene they found the Milky Way ruled by the Goa'uld. They saw the devastation they caused and vowed to do everything we could to ensure that it never happened again. Now now pay close attention when small civilizations begin reaching out into the galaxy.
They claim to be protectors, ensuring those who steal power can’t use it against those who are less advanced. The Tau’ri have been an interest of them for some time, as they caused the fall of the Goa'uld, the defeat of the Ori and the upheaval of the Wraith.
In their latest observations of the Tau’ri’s development, after scanning the George Hammond they noted it contained technologies of the once great Asgard people. When they first encountered the Prometheus the aliens believed they were children playing with forces far beyond their ken. They anticipated horrific ramifications, however they were surprised by Samantha’s actions. After awhile they began to notice the Tau’ri’s reliance on Alteran and Asgard technology. Though this would have labeled humans as scavengers akin to the Goa'uld and caused the aliens to restrict humanity’s access to other worlds, the aliens refrained from doing so as they saw that the Tau’ri were worthy. However they warned that they would still watch Tau’ri and any sign of corruption or abuse of power, they would take action to strip humanity of their interstellar technology, to “ensure they don’t bother the other kids in the neighborhood”. In the mean time they were not ready to fully disclose their existence or technology to the wider universe.