Where did stars, and light itself, come from? Is there a hidden sector of particles and forces called “dark energy” affecting the cosmos? “Most of the matter in our universe is invisible,” write Tracy R. Slatyer and Tim M. P. Tait in this special edition. Slatyer is an astroparticle physicist at the Massachusetts Institute of Technology, and Tait is a high-energy particle physicist at the University of California, Irvine. They've been looking for the bulk of the universe for a long time—and missing it.