Gravitational waves can be detected from the collision of massive objects in the universe, but also from much smaller objects like dark matter particles. (Credit: EPA/R. Hurt / Caltech-JPL)

In 2015, scientists made history by detecting the first gravitational waves — ripples in space-time predicted by Albert Einstein a century earlier. The waves were created by the merger of two black holes, each one much larger than the sun. And since then, the Laser Interferometer Gravitational-Wave Observ