Dyson Sphere

A Dyson Sphere (or "shell" as it appeared in the original paper by Freeman Dyson) is a hypothetical megastructure that was originally described as a system of orbiting solar power satellites meant to completely encompass a star and capture its entire energy output. Dyson speculated that such structures would be the logical consequence of the long-term survival of technological civilizations, and proposed that searching for evidence of the existence of such structures might lead to the detection of advanced intelligent extraterrestrial life.

Since then, other variant designs involving building an artificial structure — or a series of structures — to encompass a star have been proposed in exploratory engineering or described in science fiction under the name Dyson sphere. Most fictional depictions of a Dyson sphere describe a solid shell of matter enclosing a star, which is considered the least plausible variant of the idea.

While it is believed that some of these design variants are impractical, if not physically impossible, some designs do not require any major breakthroughs in our basic scientific understanding for their construction.

In Halo
In the book Halo:Ghosts of Onyx it is revealed that at the center of Onyx is a Slipspace rift that shunts those that pass through it to a minaturized Dyson Sphere that is seperated from normal space-time by advanced Forerunner Slipspace technology.

The Dyson Sphere and the surrounding planet of Onyx is literally a massive bomb shelter for the Forerunner to take shelter and survive the activation of the Halos.