Theoretically, there's no limit to how massive a black hole could grow. In practical terms, other constraints, such as the growth rate, suggest that the maximum mass a black hole could achieve within the current 13.8-billion-year lifespan of the Universe is about 50 billion solar masses.

A collision observed between two black holes, each more massive than a hundred suns, is the largest merger of its kind ever recorded, according to new research.