Hydrogen is the most-abundant element in the universe. It's also the simplest–sporting only a single electron in each atom. But that simplicity is deceptive, because there is still so much we have to learn about hydrogen.One of the biggest unknowns is its transformation under the extreme pressures and temperatures found in the interiors of giant planets, where it is squeezed until it becomes liquid metal, capable of conducting electricity. New work published in Physical Review Letters by Carnegie's Alexander Goncharov and University of Edinburgh's Stewart McWilliams measures the conditions under which hydrogen undergoes this transition in the lab and finds an intermediate state between gas and metal, which they're calling "dark hydrogen."