It is very cold in space, but it wasn’t always that way. That’s what I learned from my friend Guy Worthey, a professor of astronomy here at Washington State University.
He explained that when we talk about temperature, we are really talking about the jiggle motions of molecules, the tiny building blocks that make up everything around us. The more energetically the molecules jiggle, the higher the temperature.
When the universe was younger it was a really hot place.