Nuclear physicists studying particle collisions at the Relativistic Heavy Ion Collider (RHIC) have new evidence that particles called gluons reach a steady 'saturated' state inside the speeding ions. The evidence is suppression of back-to-back pairs of particles emerging from collisions between protons and heavier ions (the nuclei of atoms), as tracked by RHIC's STAR detector. The STAR collaboration shows that the bigger the nucleus the proton collides with, the larger the suppression in this key signature, as predicted by theoretical models of gluon saturation.