Both black holes were detected when they drifted in front of more distant stars, distorting their light, much as a raindrop on a window distorts background light. Astronomers call this phenomenon gravitational lensing.

In each case, the intense gravitational field of the black hole bent the light of the background star and magnified it, causing it to briefly brighten. Bennet's team was searching for these occurrences, called gravitational microlensing events. Of the six microlensing events they detected, two are thought to be caused by black holes.

Before and after images of the first event, made with the 0.9-meter telescope at Cerro Tololo Inter-American Observatory and the 1.9-meter telescope at Mount Stromlo Observatory in Canberra, Australia, show the subtle brightening of the single star. Follow-up images made with the Hubble Space Telescope revealed the true brightness of the star, something ground-based telescopes could not do. Careful analysis of the two events reveals that the lensing objects are each six times as massive as the sun but are only about 40 km in diameter.

The discovery means other maverick black holes may be present in space. If so, scientists would then assess whether they contribute to the galaxy's dark matter content. Astronomers think over ninety percent of the universe is comprised of dark matter but so far, the mysterious substance can only be detected indirectly by its gravitation effects.