Dyson sphere

Named for 20th-century physicist Freeman Dyson, who postulated a sphere could be built around a star with inhabitants living on its interior wall.

In 2369, the Enterprise-D discovered a sphere, along with the downed U.S.S. Jenolen, which crashed onto the sphere's surface. The ship carried retired Starfleet engineer, Mr. Scott.

This sphere was built around a G-type star, and measured some 200 million km in diameter, with an interior surface area equivalent to over 250 million Class-M planets. Its makers were never identified.