If space is a vacuum, how is the temperature in space on average °2.7 Kelvin and not °0 Kelvin?

Question

If space is a vacuum, how is the temperature in space on average °2.7 Kelvin and not °0 Kelvin?

In: 4

Answer 1

While there are random particles floating around space, the temperature of space being 2.7 Kelvin is literally space glowing in heat leftover from the big bang. This heat is felt as radiation – specifically microwave radiation – which are photons just like infrared light or visible light. These photons have no problem traveling through a vacuum… and the ones we’re talking about have been traveling for almost 14 billion years.