Why is space cold if there’s no matter in it?
In: Physics
Most answers here are plain wrong. Space being cold is entirely unrelated to it being mostly empty. Temperature is, informally, the average speed the molecules move at; the amount of said molecules is irrelevant.
So, why is space cold? The main reason is that it is not; humans and the things around us are just pretty hot. The average temperature of the universe is much lower than Earth or the Sun might suggest. Those two are just exceptions, which we rely on to survive.
But lets also talk about what happens if you would be dropped into space without protection from temperature (while somehow still being able to breath and not in vacuum). First, this depends on where “space” is supposed to be here: if I drop you “near” the sun in astronomical terms, lets say around Earth’s orbit, then one side of you will actually heat up quite a bit due to the Sun. The other one will slowly cool down due to thermal radiation leaving your body (this also happens on the solar side of you, but is totally overwhelmed by the sun). In short, one side gets cooked, the other frozen, but slowly. If you are far away from the sun (say, Pluto), then both sides do not get much sunlight and cool the same.
If somehow you have no protection from vacuum on your skin (yet do not die from lack of breathing), then another effect kicks in and cools you down much more rapidly: water evaporating. Just like wet skin or clothes on a hot day, water cools you when evaporating. However, in vacuum, water starts boiling already at 0°C! Thus your surface is more like a hot plate rather than just a bit warm, relatively speaking. Evaporation is extremely rapid under those conditions, untill the water gets down to 0°C (it actually starts freezing when left in vacuum, even in a chamber on earth that normally has room temperature). If you had a drop of water (or e.g. blood or any other liquid that mostly consists of it) on your skin, it would even look like water on your oven. But note that your skin substitutes for some pressure, i.e. holding blood and sweat in, thus making your insides not boil, only liquids that leave your body.
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