The crystal sphere is a prodigiously huge, not-entirely-euclidean (and thus not literally precisely spherical, at least not from within) megastructure which encloses the Astral Plane and all the planes within it. The sphere wall has no gravity and appears to be impossible to damage by any normal or magical means.
A DC30 Knowledge(The Planes) check reveals:
Openings in the sphere wall called "portals" allow ships or creatures to pass through and enter or exit from the crystal sphere. Portals can spontaneously open and close anywhere on the sphere wall.
Ships known as spelljammers sail the phlogiston outside the crystal sphere, from world to world. Spelljammers occasionally, but very rarely, visit this world -- it is considered a backwater among worlds, a long distance to sail for very little benefit, lacking much in the way of land, natural or manufactured resources, and knowledge. Knowledge has not yet percolated widely among other worlds that the Inundation is over here and land has returned to this world.
If there's a bright center to the multiverse, you're in the setting that it's farthest from.