World: Cosmology
The universe is a swirling mass of primordial chaos, within which bubble-worlds float. Weak points between the world exist where those bubble-worlds intersect, and when portals are opened, the bubble-worlds are effectively pinned together.
Living in the chaos are entities called the Elder Gods. Some are older than the bubble-worlds and they are immensely powerful; huge creatures of vague and horrifying form.
The bubbles are flattened spheres, and can be several thousands miles across. The skin is of some tough, plastic-like material not made of conventional matter, more like a meniscus on the surface of the chaos. It is soft, and feels both hot and cold at the same time. It is transparent; beyond it you can see stars. You would need something with a chaophilic surface to actually penetrate the skin.
When the bubbles formed, they were extremely hot. As the chaos cooled, so the bubbles cooled, the molten rock solidifying especially on the bottom and all around the sides. Lighter materials that had floated on top cooled more slowly. The end result was usually a world with rocky mountains around the outside of a huge sea.
World bubbles are not stationary, they float along their own prescribed courses, and seasons depend on where the bubble is within the primordial chaos. When two bubbles are connected, they adopt a new course together, so the period of season changes. Different bubbles have different day-night periods, but all connected bubbles (i.e., any the characters can get to) have the same seasons.
Stars do not move in a day-night cycle; they move only slightly as the seasons pass.
Down exists outside the bubble worlds; down is the same direction for all bubbles.
It may be that the bubble-worlds are the husks of dead elder gods, or perhaps the dead eggs from which an elder god failed to emerge.
Dragons discovered the universe, entering one bubble world, and claimed it for their own (they claim they created themselves). They brought with them numerous other species, both plant and animal, including a race of servitors. They devised a magic ritual that would, once per day, create a fiery orb that would travel across the sky, perhaps mimicking the sun of their original world.
The dragons spread to other bubble-worlds and over thousands of years the majority of bubbles became inhabited. In many worlds, the inhabitants were modified by the magicks of the dragons, more suited to the peculiar nature of that world (or the whim of the dragon).
Then suddenly the dragons disappeared. Civilisations fell in every bubble-world without the dragons to direct it. It was not like the dragons had done the actual work, but far more of an issue was the precautions the dragons had taken to keep their slaves controlled - systems designed to fail catastrophically if the dragons were not there.
The most important thing was to keep the sun manifesting. If the ceremony stops, they will never be another dawn. Each bubble needs its own ceremony, and in some instances, the knowledge was lost, and these are now in permanent twilight. But in most, the ritual was continued, allowing the day-night cycle to continue, plants to grow, life to go on. This often launched religion.
Civilisations arose again, and the weak points between worlds rediscovered. Permanent portals were put in place to keep the ways open, allowing commerce and communication.
The acts of making a portal effectively pins to bubble-worlds together. If the portal is permanent, the worlds will be stuck permanently. There is now a huge mass of intersecting bubble-worlds pinned to each other, having a deleterious effect on the chaos, annoying the entities that live there...