Suppose a planet/space station is a hollow sphere, and in the center of this sphere is a smaller sphere. This sphere would be solid, with grass and dirt like Earth. The only thing holding this smaller (but still big enough for people to live on, with a circumference of a few miles) sphere are 6 beams, attached to the inside of the outer sphere.
Civilization is mostly on the outside of this sphere. However, a small group of people live on the smaller sphere, without knowledge of this outer civilization. This smaller sphere has basically no technology at all, and must live off of what they have.
They are on this small sphere for a purpose – but what?
I have considered a few ideas. Maybe this sphere is actually a large generator that powers the outer dome, with electricity sent through the beams. It can also be a gravity generator or a gyroscope. The people there would basically be the "maintenance crew" for the sphere.
I don't know how well these would work, because maintaining an engine with basically no technology or knowledge of it does not make sense.
The smaller sphere is obviously of critical importance, and an explanation to why the people who live on it are isolated would also help the storyline.
What would this small sphere be for?
On the underside of the larger dome are lights that allow crops to grow, that dim and brighten according to a 24 hour cycle.