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")