How can we measure the temperature in outer space?

The term outer space was first recorded by the English poet Lady Emmeline ... The current black body temperature of this photon radiation is about 3 K (−270 ... Ion Imager (an instrument that measures the direction and speed of ions), ...

http://www.google.com/url?sa=t&source=web&cd=2&ved...

If you would've spent 30 seconds googling you would've found something like this:

"All of the observable Universe is filled with photons that were created during the Big Bang, which is known as the cosmic microwave background radiation (CMB). There is quite likely a correspondingly large number of neutrinos called the cosmic neutrino background. The current black body temperature of this photon radiation is about 3 K (−270 °C; −454 °F). Some regions of outer space can contain highly energetic particles that have a much higher temperature than the CMB."

the cosmic background radiation, yes.

"The color temperature of the photons has continued to diminish ever since; now down to 2.725 K, their temperature will continue to drop as the universe expands."