At last we know just how much the cosmos weighs. The answer shows that theories of the Universe's origin are spot on, says cosmologist Jeff Peterson. Trouble is, we still haven't a clue what most of the stuff is made from. How do you weigh the Universe? Astronomers have been asking this question for decades, and using every trick they can think of to get at the answer. Frustratingly, the results never added up. Different techniques gave different answers. Now a new cosmic weight-scale has been pressed into service to try to resolve the conundrum. It's the faint afterglow of the big-bang fireball in which the Universe was born. This glow can still be seen in every part of the sky. Map its structure, the idea goes, and you can work out the cosmic mass.
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