In the book Halo: Ghosts of Onyx, it is revealed that at the core of Onyx is a Slipspace rift that shunts those that pass through it to a structured Dyson Sphere, dubbed beforehand by the Forerunners as a Shield World Installation. It is separated from a normal space-time symmetry by advanced Forerunner Slipspace technology. Although Dr. Halsey calls it the Micro Dyson Sphere, the Sphere is 300 million kilometers (or approximately two astronomical units [1 AU is 149,598,000 km]) in diameter on the inside. The Micro Dyson Sphere's star has a spectra and energy output slightly smaller than the dimensions of Sol. This was going to be the haven where the Forerunners were to seek refuge when Halo activated, but no one knows what went wrong with their alleged plan, as the expanse appeared to be completely deserted when UNSC forces arrived. In Halo 3, these circumstances were revealed to be the result of the betrayal of the Forerunner AI 032 Mendicant Bias, whose defection to the Flood forced the Forerunners to abandon their plans to evacuate into the shield installations.

Basically, the structure fulfills its function as a "Shield World" perfectly. It protects the occupants of it and gives them a huge "world" to live in. Potentially, an entire new civilization could be constructed in a Dyson Sphere.

After their arrival in the Forerunner-constructed Dyson Sphere, Dr. Halsey explains the structure. She labels it a Micro Dyson Sphere because it is in fact very small, or perceived to be small when viewed from normal space. Linda also describes it as being in the bottom of a large bowl.

The Micro Dyson sphere is located in a factually compressed Slipstream space. In other terms, any object inside might appear to be a certain size, but inside the Slipstream bubble it is much, much larger. Consequently, they never left Trevelyan, but simply entered a Slipspace bubble.