The Material World, also called the Prime Material, primarily consists of an airless void called Wildspace. Wildspace is inhabited by countless solar systems where planets like Toril orbit one or more stars. The solar systems (aka wildspace systems) orbit The Eye which is the central gravitational force in the universe.
If you were to travel to the farthest reaches of wildspace, you would encounter a silvery haze through which you can reach the The Astral Sea.