The Prime Material Plane
The Prime Material is just one plane, but it contains lots of individual worlds. A world may be only a single planet or it may be a complete system with planets, moons, asteroids, comets, stars and more. Each world is sealed like a bubble in its own crystal sphere, and that sphere is surrounded within an amber stream called the phlogiston. Isolated as their worlds are, most Prime inhabitants believe themselves to be at the center of the multiverse, and their world the only one in existence.
And here’s a word of caution to arrogant planars: each Prime world is a unique domain, with its own environment and inhabitants. Travelling to the Prime without information about the destination is akin to casually jumping through a portal into an unexplored layer of the Abyss and expecting to survive. Though planars look down upon primes and their clueless ways, the Prime Material is as wondrous and infinite a plane as any, with places to explore. The worlds and occupants of the Prime Material contrast based on the current development of culture, science, magic, and natural evolution.