When scientists spotted something wiggling in the fabric of space for the first time in 2015, it opened a new window into the universe. Those ripples, called gravitational waves, came from two black holes spiraling together and crashing into each other. Since then, instruments named LIGO, Virgo, and KAGRA have detected hundreds more of these cosmic crashes. But a big mystery remained: Where do these black holes come from in the first place?

Now, two separate teams of researchers have found an answer hiding inside the data. And it's more interesting than anyone expected.

Both teams discovered that colliding black holes don't all belong to one big group. Instead, they break into distinct subpopulations, almost like how a school might split into different cliques. Each clique seems to have its own origin story.

The first team, led by Cailin Plunkett at MIT, built a computer model that looks closely at how black holes spin. The second team, led by Sharan Banagiri at Monash University in Australia, took a different approach—letting the data reveal groups without guessing what they should find. The fact that both teams, using different methods, landed on similar results makes their findings more convincing.

What stood out most was a group of unusually heavy black holes. These cosmic heavyweights each weigh roughly 40 times more than our sun or more. Plunkett's team found that these large black holes spin fast and in random directions, which points to an unusual origin: they may have formed from earlier black hole mergers rather than from collapsing stars. Think of it like finding out someone's great-great-grandparent was actually made up of several smaller ancestors.

Banagiri's team reached the same mass threshold and also spotted high spins in this group, though they urged some caution about the exact interpretation.

Together, the findings offer some of the strongest evidence yet that some black hole collisions are "second-generation" events—meaning the black holes involved had already merged once before. This discovery could help explain how black holes grow massive enough to become the supermassive giants sitting at the centers of galaxies. As detectors grow even more sensitive, the lines between these subpopulations should become sharper, giving scientists an even clearer picture of how these cosmic collisions unfold.