The space between the worlds of the Crucible is called the Void, it retained this name from ancient times when the Hammer of Hollow Mountain, the first Void ship sailed the Void. As the Dwarves had no concept of what Space was at the time they suggested referring to ancient nautical terms. Thus the Void is more formally known as the Great Void Sea, and Void ships sail it, albeit without winds. The Void is not entirely empty, some people have made a living mining asteroids and gathering gas clouds, there are also rumors of creatures in the depths of the void but none have ever been sighted.
The infinite space between the planets, incredibly hostile to life.
Various asteroid fields full of minerals and ice.