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.
Paczyński3suggested 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.
