Number 172 (Story #1), April 7, 1994 by Phillip F. Schewe and Ben Stein THE COLD DARK MATTER (cdm) theory attempts to account for the observed rotation of spiral galaxies and interactions between galaxies by invoking the presence of considerable amounts of nonluminous, slowly-moving particles. Some experiments have sought to observe cdm particles (lurking in a halo around our galaxy) either directly in detectors or indirectly by measuring the presumed gravitational microlensing effect of such dark matter (at least that part of it consisting of baryons) on the starlight coming to us from the Large Magellanic Cloud, the nearby satellite galaxy. Evalyn Gates and Michael Turner of the University of Chicago work with a model in which the dark matter near the Milky Way resides in a roughly spherical halo. They calculate that the cdm halo has a density of at least 10**-25 g cm**-3. They also assert that current microlensing experiments cannot sensitively probe the local cdm density. (Upcoming article: Physical Review Letters, 18 April 1994.)