How do scientists measure the temperature of the Universe?

The Universe used to be thousands of degrees Celsius hotter. But how do researchers measure the temperature of objects millions of light-years away to work that out?

Alan Duffy, an astronomer from Swinburne University of Technology, explains how it works in this mind-blowing video. It's like time travel, but with a thermometer.

The video also offers one of the best explanations of stars' colour-changing heat that we've seen.