Outer space, or simply space, is the void that exists between celestial bodies, including the Earth.
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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. Observations have now recently proven that it also contains dark matter and dark energy. The baseline temperature, as set by the background radiation left over from the Big Bang, is only 2.7 kelvin (K). Plasma with an extremely low density (less than one hydrogen atom per cubic meter) and high temperature (millions of kelvin) in the space between galaxies accounts for most of the baryonic (ordinary) matter in outer space; local concentrations have condensed 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.
There is no firm boundary where space begins. However the Krmn line, at an altitude of 100 km (62 mi) 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 the peaceful uses of outer space have been drafted by the United Nations, but these have not precluded the deployment of weapons into outer space, including the live testing of anti-satellite weapons.
Humans began the physical exploration of space during the 20th century with the advent of high-altitude balloon flights, followed by.