Outer space, or simply space, is the void that exists between celestial bodies, including the Earth.It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos. Theory suggests that it also contains dark matter and dark energy. In the space between galaxies, matter density can be as low as a few atoms of hydrogen per cubic meter. The baseline temperature, as set by background radiation left over from the Big Bang, is only 3 Kelvin; in contrast, temperatures in the coronae of stars can reach over a million Kelvin. Plasma with an extremely low density and high temperature, such as warm-hot intergalactic medium and intracluster medium, accounts for most of the baryonic (ordinary) matter in outer space; local concentrations have evolved into stars and galaxies. Intergalactic space takes up most of the volume of the universe, but even galaxies and star systems consist almost entirely of empty space. As of yet, space travel has been limited to the vicinity of the Solar System; the remainder of outer space remains inaccessible to humans other than by passive observation with telescopes.
There is no firm boundary where space begins. However the Kármán line, at an altitude of 100 kilometres above sea level, is conventionally used as the start of outer space for the purpose of space treaties and aerospace records keeping. The framework for international space law was established by the Outer Space Treaty, which was passed by the United Nations in 1967. This treaty precludes any claims of national sovereignty and permits all states to explore outer space freely. In 1979, the Moon Treaty made the surfaces of objects such as planets, as well as the orbital space around these bodies, the jurisdiction of the international community. Additional resolutions regarding outer space have been drafted by the United Nations, but these have not precluded the deployment of weapons into outer space.
Typically, outer space is defined based on our own perspective here on planet Earth as the area beyond the atmosphere of Earth. However, thinking on a universal scale, outer space would be the large void which occupies the (relatively) empty areas of the universe outside the atmosphere of any planet, star or other celestial body.