This pie chart is a rather prosaic representation of a truly impressive accomplishment: an inventory of the relative amounts of the different substances comprising our universe. Yellow is ordinary matter — atoms, molecules, dust, stars, planets, both visible and invisible — or what cosmologists call “baryons” (since most of the mass of ordinary matter comes from the protons and neutrons inside atomic nuclei, and protons and neutrons are classified by particle physicists as baryons). Baryons...