More knowledgeable, less illuminated

By DATA TEAM

FOR centuries, astronomers thought the cosmos was made up entirely of star-stuff, with a bit of planet-stuff thrown in. But by the 1930s it became clear that a new universal recipe was needed: galaxies appeared to be spinning far too fast, given the amount of mass apparent within them. Something invisible, but massive, was at work. It came to be known, evocatively, as dark matter. Telescopes and techniques got better, and by the 1990s, astronomers looking at the farthest-flung stellar explosions found that the expansion of the universe seemed to be speeding up. The mysterious something pushing things apart was dubbed, equally evocatively, dark energy. And there appeared to be a lot of it; as theorists weighed in, star-stuff was becoming an ever-smaller part of the universe's stuff-tally.

The leftover glow from the Big Bang, measurable by today's space telescopes, add some data to the theory. Using both, scientists reckon the recipe for today's universe is about 68% dark energy, about 27% dark matter, just a smidgen of the light, uncharged particles called neutrinos, and about 5% "baryonic" (ie, ordinary) matter. But a vast majority of that ordinary matter, perhaps 90%, is just hydrogen and helium gas floating around between stars. Only a tenth as much is actually concentrated into stars. Less than a tenth as much again—well under 0.1% of the universe—is accounted for by "heavy elements", that being more or less every bit of dust and rock in the cosmos, including the one on which you're perched.