The Planet builders are a race of extraordinarily advanced beings. They were able to create an entire star system complete with complex life on its one planet between the time the Seed ships passed its location and Destiny arrived. Very little is known about them and are only known by what the Destiny expedition found in the artificial solar system. Some, particularly Dr. Robert Caine, believed that the builders were some kind of "higher power", perhaps God, who answered to the expedition's needs, and provided them with a "lifeline" or "miracle." (SGU: "Faith")
After being wounded by a gunshot, Lt. Tamara Johansen had a dream in which she was transported to the artificial planet from Destiny and met by Caine. He claimed that the aliens brought her to the planet to save her baby, and knew everything that had happened through some unknown means. The aliens had also made a series of cabins for the settlers to survive the winter. TJ was sent back to Destiny without her baby. According to Caine, the builders were aware of her reluctant decision to return to Destiny so the civilians could stay and thus helped her by saving her baby, but she had made her choice so she could not stay. From the perspective of those on Destiny, TJ never went anywhere, and her baby died. It is not made clear if this dream was actually real, though TJ sees a Nebula from her dream in the real world which she could not have seen previously. According to Caine, this nebula had appeared in the sky of Eden just before TJ arrived and it seems to have been a sign from the builders that the experience was real as it was visible from Destiny the next time the ship dropped out of Faster-Than-Light engine. Caine seemed to be aware that it would seem to not be real, as he warned TJ that what happened will have seemed different to everyone else. According to Caine, the obelisk builders have not made actual contact, but he believes that they may still help the settlers and Destiny crew get home. (SGU: "Intervention")
However the settlers didn't receive any help and they all perished from the harsh conditions on the planet. It's possible that the Planet builders were the alien entities that temporarily reanimated the dying crew and sent them back to the Destiny in the restored shuttle, only for the crew to die off in the order that they died on the planet; one of the crew speculated that the builders had only been able to revive the crew's bodies without bringing back their souls. It was also revealed that TJ's dream was a result of the Destiny AI and not this race. (SGU: "Visitation")
Planet builder's technology
Behind the scenes
According to Joseph Mallozzi if they were able to make a third season of Stargate Universe, they would show Planet builders in it. On his blog, he stated that they envisioned the planet-builders to be an extremely advanced race who were very powerful but were very different when compared to the Ancients. They also did not possess the extensive knowledge of the Ancients nor did they evolve from a physical form similar to Humans.