A population of old black holes, 15 million light years from Earth, has been discovered by astronomers using NASA’s Chandra X-ray Observatory.

The black holes, which are more massive than any in our galaxy, can produce as much energy in the X-ray range as a million suns could across all wavelengths.

The X-ray brightness from the black hole cluster’s galaxy has increased at least 3,000-fold in the past decade, a rare spike for the non-dormant black holes.