An Unknown Alien race that seems to inhabit the Pegasus galaxy in an alternate reality was seen by Lt. Colonel John Sheppard's team when they were transported to an alternate reality by the Alternate reality drive found on an alternate Daedalus which appeared over M35-117.
The unknown aliens seem to be in conflict with Earth and the Atlantis expedition. It is unknown where they originate from and if they are at war with the Wraith or how they have concealed themselves from them to progress to the technological level they are at (it is possible the Wraith do not exist in this universe. It is also possible that they are the Wraith, albeit along a different technological path). They possess powerful ships that rival those of the Tau'ri. They do however share a resemblance to the wraith most notably with their teeth and facial structure, but they do not use biological ships or technology. They seem to be very strong in that one was capable of defeating Ronon Dex in hand-to-hand combat. They also seem to be very resilient to weapons fire, as it takes several rounds from a FN P90 Personal Defense Weapon and from Ronon's Particle magnum to take one down. They have some sort of synthetic implant in the left side of their skull that can be seen flashing red by the naked eye and seem to have a weakness to bright lights.
This unknown species was encountered in an alternate reality, where they were attacking Atlantis. The alternate Daedalus was able to disable their ship before they jumped to the next reality. However, at least four of those aliens boarded the ship after one of their Fighters crashed with the Daedalus, those encountered were eventually killed.
The Daedalus later returned to the same reality and encountered the ship again, still disabled. The Daedalus had suffered heavy damage from fighting the ship's fighters but was later saved by the Atlantis expedition of that universe who helped destroy the fighters in F-302 fighter-interceptors. It is unknown what happened to the battleship after the Daedalus jumped away but it is presumed it was destroyed by the F-302s. (SGA: "The Daedalus Variations")