Outer space comprises the relatively empty regions of the universe outside the atmospheres of celestial bodies. Outer space is used to distinguish it from airspace and terrestrial locations.

Contrary to popular understanding, outer space is not completely empty (i.e. a perfect vacuum), but contains a low density of particles, predominantly hydrogen plasma, as well as electromagnetic radiation and neutrinos. Hypothetically, it also contains dark matter and dark energy.

Outer space is the closest approximation of a perfect vacuum. It has effectively no friction, allowing stars, planets and moons to move freely along ideal gravitational trajectories. However, no vacuum is truly perfect, not even in intergalactic space where there are still a few hydrogen atoms per cubic centimeter.
(For comparison, the air we breathe contains about 1019 molecules per cubic centimeter.) The deep vacuum of space could make it an attractive environment for certain industrial processes, for instance those that require ultra clean surfaces.

All of the observable universe is filled with large numbers of photons, created during the Big Bang, the so-called cosmic background radiation, and quite likely a correspondingly large number of neutrinos called the cosmic neutrino background. The current temperature of the photon radiation is about 3 K (−270.15 °C; −454.27 °F).

Many people ask that if we put a thermometer in Space with no other light or heat source around and absolutely no background radiation there, what it would read. Would the temperature be really cold or what?

Yes, it would be really cold. Temperature measures the energy per “degree of freedom” (i.e. way something can move) of whatever molecules happen to be around. So, it becomes so cold that the molecules stop all together, and then this is the “absolute zero” temperature. On the Celsius Temperature Scale (i.e. water freezes at 0, and boils at 100), this takes place at -273 degrees C.

We usually use the Kelvin temperature scale, where Zero Kelvin is this “absolute zero” temperature -- or -273 degrees C. Water freezes at +273 Kelvin and water boils at +373 Kelvin.

If we put a thermometer in darkest space, with absolutely nothing around, it would first have to cool off. This might take a very long time. Once it cooled off, it would read 2.7 Kelvin. This is because of the “3 degree microwave background radiation”. No matter where you go, you cannot escape it -- it is always there.

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