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J. Xavier Prochaska, University of California, Santa Cruz and Jean-Pierre Macquart, Curtin University

(THE CONVERSATION) In the late 1990s, cosmologists made a prediction about much ordinary matter there should be in the universe. About 5%, they estimated, should be regular stuff with the rest a mixture of dark matter and dark energy. But when cosmologists counted up everything they could see or measure at the time, they came up short. By a lot.

The sum of all the ordinary matter t...