The Universe's Biggest Black Holes Aren't Born, They're Built

When a massive star dies, it can leave behind a black hole. That much has been understood for decades. But the most monstrous black holes in the universe, the heavyweights detected by the faint ripples they send through the fabric of space and time aren't born that way at all. According to a new Cardiff University study, they're built through repeated, catastrophic collisions in the most densely packed star clusters in the cosmos.