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. Observations and theory suggest 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 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 in the
space between galaxies accounts for most of the
baryonic 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.