EVIDENCE FOR GRAVITATIONAL MICROLENSING BY DARK OBJECTS IN THE GALACTIC HALO

THE flat rotation curves of spiral galaxies, including our own, indicate that they are surrounded by unseen haloes of 'dark matter'1,2. In the absence of a massive halo, stars and gas in the outer portions of a galaxy would orbit the centre more slowly, just as the outer planets in the Solar System circle the Sun more slowly than the inner ones. So far, however, there has been no direct observational evidence for the dark matter, or its characteristics. Paczynski3 suggested that dark bodies in the halo of our Galaxy can be detected when they act as gravitational 'microlenses', amplifying the light from stars in nearby galaxies. The duration of such an event depends on the mass, distance and velocity of the dark object. We have been monitoring the brightness of three million stars in the Large Magellanic Cloud for over three years, and here report the detection of two possible microlensing events. The brightening of the stars was symmetrical in time, achromatic and not repeated during the monitoring period. The timescales of the two events are about thirty days and imply that the masses of the lensing objects lie between a few hundredths and one solar mass. The number of events observed is consistent with the number expected if the halo is dominated by objects with masses in this range.