The Kármán line, an altitude of 100 km (62 mi) above sea level, is conventionally used as the start of outer space in space treaties and for aerospace records keeping. Outer space does not begin at a definite altitude above Earth's surface.

Most of the remaining mass-energy in the observable universe is made up of an unknown form, dubbed dark matter and 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. Local concentrations of matter have condensed into stars and galaxies. The plasma between galaxies is thought to account for about half of the baryonic (ordinary) matter in the universe, having a number density of less than one hydrogen atom per cubic metre and a kinetic temperature of millions of kelvins. The baseline temperature of outer space, as set by the background radiation from the Big Bang, is 2.7 kelvins (−270 ☌ −455 ☏). Outer space is not completely empty it is a near-perfect 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. Outer space, commonly referred to simply as space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. The layers of the atmosphere are drawn to scale, whereas objects within them, such as the International Space Station, are not.

The Kármán line at an altitude of 100 km (62 mi) is shown.

The interface between Earth's surface and outer space.

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