Rather than being composed of anything, outer space is loosely defined as the void between the planets, stars and other objects in the universe. While not a perfect vacuum, scientists consider outer space to be a hard vacuum, with a pressure level close to zero. Nevertheless, space is not completely devoid of particles, as a low density of free-floating hydrogen, oxygen, neon and helium atoms are present.
NASA’s Interstellar Boundary Explorer is a satellite that detects and measures free-floating atoms in space. It has found that the ratio of atoms in the solar system is different from outside the solar system. Inside the solar system, there are 111 oxygen atoms for every 20 neon atoms. By contrast, outside the solar system there are only 74 atoms of oxygen for every neon atom.
There is no clear boundary between Earth’s atmosphere and space. NASA considers Earth's space boundary to reside 76 miles above the surface of the planet. The United States government considers anyone who flies over an altitude of 50 miles to be an astronaut.
In the late 1800s, scientists hypothesized that a substance called aether permeated the universe. However, after numerous tests failed to find evidence of such a substance, the hypothesis was discarded.