Wildspace
Wildspace is a term slash region in the Dungeons & Dragons cosmology that originates from Spelljammer. Wildspace is a general term used to denote the "outer space" regions of any and all Crystal Spheres, distinct from the Phlogiston, which is the plane/medium used to travel between Crystal Spheres.
The term went underground with Spelljammer during Dungeons & Dragons 3rd Edition and Dungeons & Dragons 4th Edition, but made a return in Dungeons & Dragons 5th Edition, with some subtle yet distinct changes to its place in the overarching cosmology. In 5e, Wildspace still refers to the airless void in between the planets and stars of a given solar system, but it is now characterized as a kind of "interim zone", where the Prime Material and the Astral Sea overlap. This gives it certain characteristics in common with the Astral, but not enough to fully bring over that plane's more extremely magical attributes - for example, there is no freedom from aging or the need for eat, drink and sleep in Wildspace. The Crystal Spheres concept is gone; now, if you just sail beyond the "bubble" of Wildspace surrounding a given world, you eventually slip free of the Prime Material entirely and into the Astral Sea. In 5e, the term "Astral Plane" refers to Wildspace and the Astral Sea as a whole, with the Astral Sea further corresponding to the "Deep Astral" of the classic Planescape lore.
While we say the Crystals Spheres are gone, one is briefly mentioned in the adventure that accompanied the book. Here it is presented as having been created by the Primordials when they managed to expel the gods from the system (called Doomspace, it's pretty rad, with a magical black hole instead of a sun). At some point the gods managed to break the sphere and return to the system, but no one knows how they did it, and they may have caused the sun to turn into a black hole in anger at being denied worship by the mortals in the system.