Searching for a black hole no bigger than Sydney s CBD across hundreds or thousands of light years of space demands a sneaky approach. First you have to find a visible star that a black hole has trapped in orbit. Then you have to study how the star wobbles. John Wheeler described it as like looking for a pair of dancers on a dark dance floor. The heavy man dressed in black is invisible, but the bright white dress of a light women is an easy target as she is whirled around.
Astronomers look for the bright stars that orbit dark partners in the same way.
One of the best candidates is the star called V404 Cygni. Calculations shows V404 s dark partner is twelve times more massive than our Sun, yet totally invisible. But for every black hole orbiting another star, there must be many more solitary ones yet unseen. One of these could lurk quietly much closer to home. Although black holes have the power to hoover up anything and everything that strays too close, they can t hunt.