# What's the avarage temperature of the Universe?

I'm sorry if this was asked before but with all the stars and dark matter and all the other stuff, curently in the Universe, what's the avarage temperature of the Universe? Is it like extremely high or extremely low?

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It depends on how you want to average:

If you average by volume then all you need to think about is the bath of photons from the cosmic microwave background, at about 2.7 Kelvin.

If you average by observable mass then the primary constituent is sparse atomic (H) and molecular (H2) gas in the voids between galaxies -- at a temperature of a few 10s of Kelvin. Only about 1/10th of that mass is in stars, but their temperatures are far higher---closer to $10^4 - 10^5$ Kelvin---so once you average that, you'll get something more like $10^3 - 10^4$ Kelvin.

Finally, if you average by total mass---predominantly Dark Matter which is in virial equilibrium in most galactic halos, then the temperature would be way up to about $10^7$ Kelvin.

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You can measure the temperature by looking at the spectrum, and in this case you see a peak at 2.73 K. There are about $10^8$ times more CMB photons than starlight photons in the Universe at the present epoch.