Number 256 (Story #1), January 26, 1996 by Phillip F. Schewe and Ben Stein MACHOS MAY MAKE UP A CONSIDERABLE FRACTION OF DARK MATTER in our galaxy's halo, a new study shows. Last year the MACHO (massive compact halo object) collaboration announced the observation of three events in which the light from a star in the Large Magellanic Cloud brightened as a result of gravitational microlensing by a nonluminous object lying in the galactic halo along our line of sight. At last week's meeting of the American Astronomical Society in San Antonio, David Bennett of Livermore announced that the MACHO team has now recorded seven events. Since their report last year, the team has improved the sensitivity of its detection process, both of lighter halo objects (down to 10**-6 solar masses) and heavier objects (1 solar mass). The greater mass sensitivity was decisive: the seven events appear to correspond to white dwarfs in the range 0.1 to 1.0 solar masses. The MACHO group finds these needles in a haystack by taking pictures of the 9 million stars every night and looking for telltale light enhancements. The event sample is not very large, but the researchers are so confident of their events that they use the number to estimate a value of at least 50% as the likely contribution of MACHOs to the dark matter believed to be lurking in the halo.