Jenolan Dyson Sphere

A Dyson Sphere is a construct whose inner surface is approximately 1 AU from a star (the entire structure being a little more than 2 AU in diameter). This allows for a very large surface area capable of supporting life. Gravity (through the mass of the structure, or by artificial mechanics) would essentially pull everyone 'down' towards the hull and away from the star. Topsoil, and atmosphere would most likely be present between the hull of the sphere and the star. 

Such a structure as a Dyson Sphere would have an internal surface many times the size of all 24th century Federation planets combined. The Federation did not have the technology at the time to create such a construction.

History
The Dyson Sphere was first theorized by Freeman Dyson.

In 2269, the USS Enterprise located a Dyson sphere called Lyra headed towards a black hole. 

Federation astronomical surveys received infrared signatures that were presumed to be from Dyson Spheres. 

The USS Jenolen crashed on the surface of a Dyson sphere in 2294. In 2369, the USS Enterprise (NCC-1701-D) discovered the Dyson sphere. 

The Dyson sphere discovered by the Enterprise was located in the Beta Quadrant. 

Miles O'Brien underwent a telepathic "engineer's nightmare" in which he wandered along a sandy desert surface inside one. As for what seemed months, if not years. This illusion was brought on by the Pah-wraith aliens that targeted his fragile, vulnerable psyche from his previous Argrathi incarceration. (Star Trek: Deep Space Nine Millennium; )

One type of Dyson Sphere would appear to be of different dimensions, as when one falls from the 'top' of the inside of this sphere, gravity pulls that person to the 'bottom' of the sphere rather than to the hull. Also, in this case, a Romulan cloaking device may have been used to hide the sphere, but the gravity well it created would be quite noticeable. 

The USS Titan stumbled upon the planet of New Erigol, which the inhabitants had attempted to hide using a Dyson sphere like construct around both the planet and the star. Although the covering eliminated visible output from either, it still allowed high energy pulses to escape, allowing for its detection.