There's a serious undercount between how many black holes we've found out in the cosmos, compared to how many are expected to exist. But that's not too surprising really – these are the darkest objects possible, set against the inky void of space. Many black holes make their presence known through other means, such as hot bright disks of material orbiting them, flashes of light as they gobble up stars, or gravitational waves as they collide. But it's long been predicted that many more are lurking out there far from other objects, essentially invisible.

There is however one way that these isolated black holes could reveal themselves – gravitational lensing. As they move in front of a background light source, the intense gravity warps that light ever so slightly, brightening and shifting it.