The sun warms up our planet but space is cold. If I lit a candle and stood 20 feet away, I could see the light but not feel the warmth. So, why do we feel warmth from the sun but space isn’t warmed by it?
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Answer: temperature is both a measurement of energy and of pressure. Let’s imagine we have a bunch of O2 atoms in a 1m^(3) box and we want to know the temperature of that box. First we might do some measurements to figure out how much energy each atom is carrying. The more energy they carry, the more often that they will collide with each other and the box. The more often these collisions happen, we call that “hot”. Vice versa for “cold”.
Next we need to know the pressure inside that box. This is easy to calculate because it’s just the energy of those atoms in a given volume and we already know both those values. If we were to make the box smaller, the temperature would go up because those atoms are in a smaller space so they collide with each other and the box more often. If we make the box larger, the temp will go down due to fewer overall collisions.
What does this have to do with outer space? Space is the biggest box the universe has to offer and it is 99.9% empty. So we have a massive box that has very little “stuff” inside of it so of course outer space is cold!
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