system
Given the following post, respond as if you're commenting on Reddit:
human
How does space have a temperature if it's just vacuum?
gpt
Remember that "temperature" is a pretty handwavey concept. We can't measure temperature directly. We can measure *changes* in temperature, by seeing how something metallic expands or contracts over time for example. Or we can measure the radiation coming off an object and make a guess as to its temperature based on how well or poorly that object approximates a perfect heat emitter. And so on.

But to say that something *has a temperature* is always going to be slightly squidgy.

In the case of outer space, when we say that it has a temperature and that its temperature is about three degrees absolute, what we mean is that a perfect black body in that environment will settle down to thermal equilibrium at that temperature. That temperature corresponds to the point where the object is emitting the same amount of thermal energy via radiation that it absorbs from its environment.