Re: What temperature is a complete vaccuum?

If you stuck a thermometer into a perfect vacuum, it would 
eventually cool down to absolute zero (0 degrees Kelvin, 
-273 degrees Celcius, and -459 degrees Farenheit), so I would say a 
vacuum had a temperature of absolute zero.
But you could never find or make a vacuum that perfect.

Even if you could remove every single
atom of gas from a box, it would rapidly fill
with photons emitted by the walls of the box.
The temperature of the photon gas would be 
the same temperature as the walls of the box.

As for outer space, even light years away from all stars 
and gas clouds, the vacuum is filled with a
gas of photons at 3 degrees above absolute zero.
This is the last light of the big bang, 
discovered in 1964 by scientists trying to test a 
very sensitive radio antenna. The density of this gas is 
200 million photons per cubic meter. Not much of a vacuum!

You might also want to see this question from the MAD Scientist archives:

Is 0K absolute zero and does absolute zero exist?