CAMBRIDGE, Mass., June 5 (UPI) - President Clinton (Friday) welcomed the discovery by researchers in Japan that neutrinos, the so-called dark matter in the universe, actually have mass. Delivering the commencement address at the Massachusetts Institute of Technology, Clinton called the neutrino discovery another key step in understanding how the world works, and said the Japanese success ``calls into question'' the U.S. government's decision to abandon the Superconducting Super Collider project for examining the tiniest elements of the universe.