Space is a massive expanse of weightless vacuum. Within this colossal expanse sit planets orbiting stars, which in turn slowly orbit to form the galaxy. In this way “space” in the Cosmonomicon setting is very much like space in the real world. However, the details are not the same. Stars are burning spheres of fire connected to the elemental plane of fire. Planets and moons are likewise made of the combination of 4 elements: fire, water, earth, and air. The hand of creator gods can be seen in their formation. Particularly striking is the abundance of life populating the many habitable worlds. Nearly every large asteroid, moon, or planet is at least able to support life, with reasonable gravity and a breathable atmosphere. Most of these worlds actually contain life, and that life is similar from world to world and solar system to solar system.
Planets in setting are often largely of a single biosphere: desert worlds, forest worlds, water worlds dotted with islands, etc. Such “onenote” worlds tend to be dominated by only a few intelligent races that are best adapted to that environment. A forest moon, for example, might be home to just fey and goblinoids. There are exceptions of course: some 10% of planets host complex environments like those found in most major D&D settings.
Gravity and Atmosphere in the setting are largely simplified to encourage easy gameplay and to mimic the tropes of old Sci-Fi: sufficiently large objects in space, starting with big asteroids and ranging up to planets more massive than Earth, possess gravity similar to Earth. Low gravity worlds might be closer to Mars than the Moon, while large dense planets might be a bit “heavier” than Earth, but they are all readily available to adventure on. Similarly, these bodies possess breathable atmospheres thick enough to at least be somewhat comfortable to humanoids. Particular worlds might be a little thin and cold, or dense and heavy, and it’s not uncommon for particular worlds to smell differently, or even to be dangerously toxic or acidic, but they seem to always contain enough oxygen to breath and be thick enough to not require suits.
Metaphysically, the cosmology of the setting is still the default of D&D: The Great Wheel (PHB p300, DMG p44). The galaxy and the wider universe are all the material plane. The inner and outer planes also exist, just as described in the core D&D books. Similarly, the material plane also possesses its echoes: the Feywild and Shadowfell. That means every planet, moon, asteroid, and star has it’s own echoes in those planes. In the empire, these dimensions are sometimes called Shadow-Space and Fey-Space. Space travel is not especially common in these dimensions, largely because no crystal gates link solar systems in these echo dimensions Strangely, the layout of stars on any given world does not match those in fey-space implying that while every world has a mirror in these other dimensions, the galaxy is not laid out in the same fashion. Stars in Shadow-space do not burn with any light, and therefore no sun nor constellations are visible.