"What is ?"

CMB

I thought you'd never ask!

Page 3

It is perhaps the most important key pieces of evidence into figuring out about the universe. Arno Penzias and Robert Wilson accidentally discovered this, and boy, they sure stumbled upon something great. It is electromagnetic radiation that fills our universe. It shows us that the Universe at one point was extremely hot and has cooled down tremendously since. And we can relate this to our universe expanding, because the CMB is the same temperature everywhere.