The average temperature of deep space is approximately -270.45°C or 2.73 Kelvin), which is just above absolute zero. This baseline temperature is set by the Cosmic Microwave Background (CMB) radiatio...
Which is absolute nonsense, because vacuum has no temperature.
https://en.wikipedia.org/wiki/Black-body_radiation
It has nothing to do with the movements of atoms, but just with the spectrum of photons moving through it. It means that eventually, any object left in space will reach that temperature. But it will not necessarily do it quickly, which is what you need if you're trying to cool something that is emitting heat.