Beginning in 2018, one of the brightest X-ray lights in the sky went dark, and scientists still aren’t sure why. 

The black hole responsible for creating the lights-out mystery lives in GRS 1915+105, a star system 36,000 light-years from Earth containing both a normal star and the second-heaviest known black hole in the Milky Way. That heavyweight is 10 to 18 times the mass of the sun and second in mass only to Sagittarius A* (or SgrA*), the supermassive black hole in the galactic center. The region around the GRS 1915+105 black hole typically shines with an intense X-ray light, as it feeds on its companion star. As the material circles the cosmic drain, the particles within rub together, generating energy before dropping into the darkness at the black hole’s center. That swirling material is the black hole’s accretion disk, which lights up with X-rays as the black hole devours more and more sustenance.