When two black holes orbit each other and merge, they send gravitational waves across the universe, rippling spacetime itself. LIGO and Virgo detectors have observed these waves directly since 2015.
It matters because gravitational wave detection opens a new way to study the cosmos.
It also confirms predictions of Einstein’s general relativity in extreme conditions.
Merging black holes create ripples in spacetime that can be detected billions of light-years away.
[LIGO, ligo.org]