What is the average temperature of mass in the universe? That is, ignoring speculative concepts like Dark Matter and just summing across stars and gas clouds etc
 A: This is a somewhat odd question since average is a somewhat vague term.  Are you referring to temperature averaged by total mass? Or average temperature by volume?
As mentioned in the comment above, the most important "Universal temperature" is the temperature of the Cosmic Microwave Background which is basically the light that was released when the universe cooled enough for electrons to be captured into atoms.  At this point the universe became transparent the first time.  As the universe expanded, the energy density dropped and the photons were stretched until they've moved into the microwave spectrum.  This temperature corresponds to 2.73 K.
However, photons aren't massive, so they don't really fit as matter.  Another important Universal temperature is the Cosmic Neutrino background.  Along with the photons, neutrinos are everywhere throughout space.  They are slightly cooler than photons because they could not couple with the energy produced during the electron-positron annihilation epoch.  The CNB temperature is 1.95K.
These are probably the two most important universal temperature measurements we have.
