system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If space is a vacuum, how come it's so cold?

Wouldn't there have to be something there to be cold? What's making it cold?
gpt
Unfortunately, space isn't just space. How "hot" or "cold" it is depends on where you are. It also doesn't help that heat "moves" differently in space than it does on earth due to the vacuum.

If you have a thermometer in your house and take it outside, it will warm up or cool down until it reaches the same temperature as everything around it, with "everything" mainly being the atmosphere or whatever rock/table/bit of ground you left it lying on. By coming in contact the atmosphere and the ground, the thermometer can easily exchange heat with the environment and will soon give you an accurate reading of the temperature.

Now let us imagine you're on the International Space Station and you do the same thing. You take your thermometer out of the ISS and leave it floating a little bit outside the airlock. Let's say you also did this "at night", i.e. when the ISS passes through earth's shadow so that the sun doesn't interfere with your experiment. The thermometer's temperature will barely change. This doesn't mean that the space is the same temperature as the inside of the ISS, it's just that your thermometer can't exchange heat the way it would on earth since it's in a vacuum. It will cool VERY SLOWLY by emitting so-called black body radiation (this is exactly the same as the light that metal gives off when it glows red hot, except that your thermometer is only at room temperature, so it only glows in the infra-red).

Now let's say you wait a little longer. The orbit will carry the thermometer out of earth's shadow into full sunlight. Now, the thermometer is giving off a little bit of IR light (which cools it down slowly very slowly) and absorbs large amounts of light from the sun (which heats it up rapidly). This means that your thermometer floating in space will get HOTTER! And for quite a while, it will continue to get hotter and hotter. However, as it gets hotter, it will emit more and more black body radiation until the radiation it loses is equal to what it gains from the sun. At this point, it will be showing the "correct" temperature, which is quite hot for the ISS. After all, when you're in orbit around earth, you receive more light per square meter than if you were sitting in the Sahara desert at mid day.

Finally, let's imagine that you did the same thing from a space ship that was floating in deep space (i.e. well outside of our galaxy). Now the thermometer will lose heat through black body radiation as it did before, but there is no sun to heat it up. The only light it can absorb is the so-called "cosmic microwave background", which is the left-over light from the big bang. But otherwise, the situation is the same. The thermometer will continue to lose temperature (and hence will emit less and less black-body radiation) until it is giving off as much as it is receiving from the CMB. This happens at about -270C, which is very cold indeed. This is why people say that space is cold, but it's really only true for deep space.