How is space “cold” when it’s a vacuum? Isn’t temperature transferred between mass? If anything, shouldn’t you overheat when in a vacuum, because your body generates heat?
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There are a few sides to this. Heat isn’t just transferred through contact. In fact, there are 3 main methods of heat transfer. But first, what do we mean by “heat” or “temperature”?
Really, temperature is just how quickly all the particles in something are vibrating, i.e. the how much the tiny stuff that makes up big stuff is wobbling. Heat is just these wobbles getting passed on to something else.
This transfer can happen through conduction, where neighbouring bits of stuff make each other wobble. If you heat one end of a metal bar, the atoms in that end vibrate quicker. They pass these vibrations onto their neighbours, who also vibrate quicker. So, the heat gets conducted along the bar. Or, we could put two different objects of different temperatures together, and the heat conducts from one to the other.
The next method is through convection. Rather than the heat being passed on between neighbours, the different particles actually move about as they vibrate. In a pot of water, the heated molecules at the bottom move up to the top, and let other particles take their place at the bottom to get heated there. Or, you could think of a mascot at a game moving around in the crowd and hyping them up.
The third method is radiation. Everything that has heat energy is giving off radiation: light which is (partly) beyond the visible spectrum. Infrared light shoots off of stuff, hits other stuff, and warms it up. That’s why it feels so hot next to a fire. The glowing hot coals/logs are firing infrared radiation at you, making your face get hot quickly.
In space, we can’t transfer through convection or conduction because there isn’t any contact. However we can still emit radiation. But, as you say, heat can still get trapped in things. So why do we say space is cold?
Well, it’s really because the stuff *in* space is cold. Space isn’t a perfect vacuum. And the particles in space are wobbling with some temperature. It turns out that the average temperature of the stuff in space is something like 3K, or -270°C. (If you want to work that out in freedom units, feel free!) That’s pretty darn cold!
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