In black holes gravity is so strong not even light can escape, making them truly dark objects that don't emit any form of radiation.
However, scientists at Cardiff University say black holes that become deformed through collisions with other black holes or stars are known to emit a form of radiation called gravitational waves, predicted by Einstein nearly a hundred years ago.
The waves are ripples in the fabric of space-time that travel at the speed of light but are extremely difficult to detect.
Sensitive instruments in the United States, Europe, Japan and India can detect gravitational waves in roughly the same frequency range as audible sound waves, and can be thought of as a microphone to gravitational waves, researchers say.
Two black holes that collide will produce a black hole that is initially highly deformed, creating gravitational waves not in one tone but in a mixture of a number of different tones, very much like the dying tones of a ringing bell, they said.
Detecting the space-time ripples of such a black hole and measuring their frequencies can reveal the mass and spin of a black hole for scientists without them going anywhere near it.
And it can reveal even more, researchers said.
"By comparing the strengths of the different tones, it is possible not only to learn about the final black hole, but also the properties of the original two black holes that took part in the collision," Cardiff researchers Ioannis Kamaretsos said in a university release.
The finding provides a way of studying the properties of the binary that produced the final black hole even when the binary itself is not visible to a gravitational wave detector, scientist said.
"We never guessed it would be possible to weigh two black holes after they've collided and merged," researcher Mark Hannam said.