Outer space is a term used to describe everything existing outside of the Earth's atmosphere. It mostly consists of empty space with clusters of stars, but also solid matter such as comets and meteoroids.
There is currently little knowledge on the subject of outer space. The basic observations of astronomy have given an understanding of planetary movements as well as basic predictions; from basic waxing and waning of the moon to the arrival of comets and eclipses.
The word "space" was specifically mentioned as the place from where the meteorite that made Toph's bracelet and Sokka's sword came. The characters demonstrated knowledge that the meteorite did not originate from Earth.