Black holes are usually detected by the effects of their vast gravitational field on the orbits of nearby astronomical objects such as stars. Naked black holes have no such material near them making it practically impossible to detect these objects. However, when a black hole drifts in front of distant star - as seen from Earth - light from the star undergoes a microlensing effect, creating two separate but close images of the star. The black hole's gravity also magnifies the brightness of the images making the passing of the black hole in front of the star easier to detect.

The two microlensing events were first discovered in 1996 and 1998 by the Massive Compact Halo Object (MACHO) collaboration. Further observations were carried out by the Global Microlensing Alert Network and Microlensing Planet Search. In both cases the two stars appeared to brighten over a period of 800 and 500 days, providing the astronomers with some estimate of the mass of the black holes. Then last year observations with the Hubble Space Telescope allowed the teams to identify the lensed stars and make more precise measurements of the stars' original brightness.