History and specifics
The Hollow Planet's surface was covered with millions of cylindrical structures that captured life energy, surrounded by concrete-like surfaces and material that appeared to be grass. Any surface disruptions were neutralized by organically-based maintenance machines.
The cylinders were constructed some 40,000 years ago by unknown beings who were in advance of Federation technology by 10,000 years, who were capable of devising a material less than one millimeter thick that could withstand large amounts of weight.
40,000 years ago, the planet was consumed or subsumed by a life energy-devouring entity that resided within its interior. Despite becoming hollow, the planetary body continued to have gravity and atmosphere comparable to class M planets.
For at least the previous millennium, the Hollow Planet zig-zagged within the intergalactic void above the galactic plane as it systematically ravaged all life energy from at least 29 planets.
In 2269, the USS Enterprise encountered the Hollow Planet after plotting and retracing its route. A landing party beamed down, which led to the death of a security officer and the capture of James T. Kirk. Spock theorized that the destructive energies of the Rohrbough Maelstrom would be capable of destroying the Hollow Planet, and the Enterprise lured it there at warp speed. Kirk was rescued, but he sustained third-degree burns and required extensive recovery in an intensive care unit in sickbay. (TOS - New Visions comic: "The Hunger")