Outer space, or simply just 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, neutrinos, dust and cosmic rays. The baseline temperature, as set by the background radiation from the Big Bang, is 2.7 kelvin. Plasma with a density of less than one hydrogen atom per cubic meter and a temperature of millions of kelvin in the space between galaxies accounts for most of the baryonic matter in outer space; local concentrations have condensed into stars and galaxies. In most galaxies, observations provide evidence that 90% of the mass is in an unknown form, called dark matter, which interacts with other matter through gravitational but not electromagnetic forces. Data indicate that the majority of the mass-energy in the observable Universe is a poorly understood vacuum energy of space which astronomers label dark energy. Intergalactic space takes up most of the volume of the Universe, but even galaxies and star systems consist almost entirely of empty space.
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