Can't ignore bosons; photon wavelength is a measure of temperature too.

Space has a temperature, which is based on the average of incoming radiation through that space; i.e. the thermal equilibrium to emit as much energy as is absorbed by a theoretical perfectly thermally conductive black body at that point in space.

Based off CMB radiation, space on average is a little over 2.7 kelvin. It'll be hotter near stars, but the void dwarfs matter on a cosmic scale