Glorantha is a mythic universe based on medieval and ancient cosmogony, not modern science. The world sits in the middle of the cosmos. Surrounding it is a sphere around the world.
The upper part of this sphere is the skydome, with stars and celestial beings. The underside of this, below the world, is hell.
All these were originally held together by a gigantic adamant pillar through all the universe’s layers – the Spike.
When the Spike was destroyed by the Chaos Rift, the world of Glorantha naturally began to fall apart.