DS-402 is a "miniature" Dyson Sphere discovered in the Delta Quadrant by a team from Starbase Phoenix in 2405.
With a diameter of 3,705 Km. it is slightly larger than Earth's moon. Unlike other known Dyson Sphere's it is not constructed around an actual star or sun; rather it is built around a reactor which serves as an artificial sun.
DS-402 is inhabited by 4 distinct races, separated geographically by large mountain ranges inside the Sphere. Each of the four cultures: The Luakans, The Manilans, The Aquatics and The Reptilians had noticeably different cultures, beliefs and technology levels, leading Starfleet scientists to believe that this was another social and genetic experiment conducted by the Builders.
In 2405 Starfleet Officers from Starbase Phoenix came across the sphere, which had apparently been damaged by some sort of collission, as it showed signs of hull breaching. As the team arrived aboard the USS Uller, a rudimentary space faring vessel exited the sphere through the breach in the hull, and made contact with the USS Uller. It was the Luakan's first expedition out of the sphere.
The team assisted in the reparation of the reactor, which was damaged and overdue for maintenance. The reactor actually gave out for some time while the Starfleet Engineers, lead by Lt. G'ldraarg, worked on repairing it. The blackout led to the death of the Reptilian race, and the downfall of the Manilan culture.
Currently a team of Starfleet scientists works in conjunction with the Luakans inside the sphere to study and learn from builder technology.