Question: What makes space so cold?

Heidi answered on 19 Nov 2019:

It’s a tricky one! We can think of this in a few ways. One way is that we need to think of hot and cold not just as temperatures, but as a measure of how fast atoms and molecules are moving. In space, there’s not many atoms and molecules, it’s mostly empty. This means there’s not many atoms and molecules to move, so the temperature is low.
Another part of the problem is the way heat is transferred. You might have heard of convection, conduction, and radiation? Well, in space, convection can’t occur where there isn’t liquid or gas, and conduction can’t occur if there’s nothing to transfer the heat, so the only way left is radiation. This is still a good method of transferring heat, but you won’t get all the heat transferred.