The Unknown race is an unknown species. They appear highly advanced technologically, having ships with high yield energy weapons. They can understand or at least translate English. They come from a different galaxy. (SGA: "Pleasure Cruise")
When the Goa'uld rise to power, the aliens are unable to stop it due to being preoccupied with problems in their own galaxy. Seeing the devastation the Goa'uld wrought, the aliens vow to do everything they can to ensure that it never happens again. As a result, they begin testing races that begin reaching out into the galaxy and restrict those who steal power to ensure that they can never use it against those who are less advanced. Typically, this involves restricting them to their own planet or star systems and crippling their interstellar capabilities. The Ancients, the Nox, the Furlings and the Asgard are aware of this race's actions and let them know if they overstep with their actions. (SGA: "Pleasure Cruise")
They made contact with the Tau'ri in 2004 when they attacked the Prometheus and captured its crew as a means of testing the Tau'ri. 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. The Prometheus crew retained no memory of what happened between the ship scanning the escape pods and being returned to the Prometheus. (SG1: "Grace")
In 2009, the aliens appeared in the Pegasus galaxy, once again testing the Tau’ri. They engaged the George Hammond under the command of now-Colonel Carter who recognized the ship from the first encounter. The more advanced George Hammond proved to be no match for the aliens who once again made the crew disappear. Confronted by illusionary figures again, Carter realized that the aliens were testing her both times. Once Carter figured it out, one of the aliens began communicating with her directly using an image of John Sheppard.
The alien explained that while they had passed the test the first time, the aliens had decided to test them again since the George Hammond was far more advanced than the Prometheus and was equipped with technology from the extinct Asgard race, something that the Prometheus had lacked. In both instances, the aliens had chosen to leave Carter behind because she was the most experienced and intelligent member of the crew and thus the most likely one to pass their test. The impact the Tau'ri were causing in the galaxy prompted the first test since the aliens had anticipated horrific ramifications but were surprised by what actually happened. However, the Tau'ri's reliance on Ancient and Asgard technology worried the aliens that they would become scavengers like the Goa'uld and in the past, such a fear would've caused the aliens to restrict them.
The alien went on to explain how their preoccupation with their own problems prevented them from stopping the rise of the Goa'uld and pointed out how the Tau'ri tear Earth apart with wars and then send their soldiers out into the galaxy with weapons. Both acknowledged that having weapons doesn't make one inherently evil which is why the aliens disabled the ships of other races, took away their weapons and saw how they react. The alien admitted that their ship getting stuck in the gas cloud the last time wasn't a part of the test: they had been trying to close the distance with the Prometheus to beam out the crew and hadn't anticipated the ship launching escape pods. The aliens almost didn't take the risk of following Prometheus into the cloud and stated that in all of the time that they had been testing races, the Tau'ri were the first to almost escape them.
The alien stated that the Prometheus almost getting away was something that she should take as a compliment and that the aliens had liked them: soldiers combined with scientists, brains and brawn protecting each other in a balance to the extremes that the aliens had found fascinating. Carter herself being a combination of both was of particular interest to them with many wondering if Carter's brains or brawn would ultimately take precedence. Carter stated that she tried to let her brains win out, but it wasn't always possible, something that the alien acknowledged was true but he felt that the true measure of a species was how it used the weapons that it did possess and noted that the Tau'ri had a peculiar history with their large scale weapons. Carter acknowledged that that extended to their own planet and asked what would have happened if they had failed with the alien pointed out that she had seen what their ships were capable of.
Carter reminded it of the brutality of such a thing and the alien defended their position and stated that his race saw themselves as protectors of those who couldn't defend themselves and that there were species out there that would obliterate and take over smaller and weaker planets just like the Goa'uld did. The alien reassured Carter that even they had allies they would have to answer to amongst the great races of the galaxy like the Ancients, the Asgard, the Furlings or the Nox. Though the aliens kept their own counsel, the other races were aware of them and would let the aliens know if they had ever overstepped. Carter questioned what the aliens did to those they found unworthy and the alien states that they make sure they can't harm others, usually by restricting them to their own planets or star systems and crippling their interstellar capabilities. Carter realized that the aliens looked for more than just their worthiness and he confirmed it, stating that they looked at people with their fingers on the trigger and test them for hope, compassion, trust and love, all things critical in someone who was entrusted with the power to act like a god.
Carter saw a vision of all of the people she cared about and entrusted her life to and when she turned back to the alien, he'd shifted into the form of General George S. Hammond. Channeling Hammond's mannerisms, the alien questioned if Carter was unsure if she belonged there and had the right to make the hard decisions. Carter expressed her fears at being in charge to the alien in the form of her friend and mentor who did his best to reassure her and reminded Carter that while he wasn't the real General Hammond, his image and mannerisms were based upon Carter's memories and perception of General Hammond. Carter questioned the point of the test and the alien pointed out that the Goa'uld had acquired power wherever they could and posed as gods and because the aliens couldn't stop it, billons suffered. As a result, whenever they found a race such as the Tau'ri on the verge of becoming great, they stepped in to test them to keep such a thing from ever happening again. When they put Carter to the test, she had sought answers to not only save herself, but the people who were taken and to seek a peaceful resolution and in so doing, passed the test. The alien promised that they don't execute anyone and if Carter had failed to find a way out of the cloud in time, she would've been beamed off of the Prometheus and deposited with the crew on a safe planet before they quarantined Earth.
The alien reminded Carter that without their technology, the Goa'uld were nothing more than bullies who had found a bigger stick than the rest of the galaxy. The aliens were making sure the Tau'ri didn't end up the same way and with time, even the best of intentions could lead to unforeseen consequences and even the most stalwart hero could be corrupted. The alien insisted that even they had others who would step in if they abused their power and there was more to the universe than Carter had ever dreamed. Carter decided to accept it even if she didn't like it and the alien concluded that the Tau'ri were doing the best they could to do good things despite some missteps and that the Tau'ri were one of the most promising races that the aliens had ever encountered. Carter recalled that an Asgard had once told Jack O'Neill the same thing and had been right. Carter attempted to get the alien to confirm or deny if his race was the Furlings, but he refused to respond either way and Carter told him that if his race ever got tired of flying under the radar, they were welcome to form an alliance as they'd obviously fit right in and would be good allies to have on their side. The alien recognized that Carter was suggesting it if only to keep an eye on them and their technology as a form of checks and balances which he accepted.
As the two shook hands, Carter requested if before he left, she could use the opportunity she had never gotten to say goodbye to General Hammond through him. The alien granted her request and returned her goodbye to Hammond before returning to the form of Sheppard to make things easier for her. The alien was unsure if they'd ever meet again, but suggested that it was possible given how far the Tau'ri had advanced and how much they had accomplished. Carter jokingly asked the alien to make sure she wasn't aboard the ship the next time they might decide to perform such a test on the Tau'ri which he accepted in good humor. Carter asked him if he could fix the ship too before he went and the alien smiled and disappeared in a flash of light that left behind the Hammond's crew. Major Kevin Marks found that the ship's engines had indeed been restored by the aliens while their ship was moving off. Before the alien ship moved off completely, Carter spotted the alien on the bridge once more in the form of Hammond. The alien gave Carter a salute before he disappeared for good as his ship moved out of range. (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. However, one refuses to confirm or deny if they are the Furlings.
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 they could to ensure that it never happened again. Now paying 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.