A Midnight Sphere was a spherical shell of gas and plasma capable of absorbing light and sensor beams, first theorized by Dr. Culla sometime prior to 2268. Additional theoretical work and calculations had been performed by another researcher, Toliver.
In 2268, Commodore B6 Blue, "Bishop", planned to create a Midnight Sphere to enclose "The Yard", a secret Starfleet facility, and recruited Spock to help do so. The resulting field encompassed a volume three-quarters that of Earth, though it also emitted hazardous plasma waves beyond the sphere's surface, making its use impractical. (TOS short story: "Devices and Desires")