For just one-tenth of a second in May 2019, the universe delivered a signal that did not fit the usual script. LIGO and Virgo recorded a gravitational wave from GW190521, but unlike the familiar rising chirps from orbiting black holes spiraling together, this one arrived more like a crack, brief, blunt, and missing a clear inspiral phase. That made it an oddity from the start. The LIGO-Virgo-KAGRA collaboration interpreted it as the merger of two black holes, one about 85 times the Sun’s mass and the other about 66 times, which then formed a remnant black hole of about 142 solar masses. In that reading, GW190521 became the first observed example of an intermediate-mass black hole. Even so, the event has never sat comfortably within standard expectations. The source black holes fell into a mass range that the article describes as being in tension with established models of stellar evolution, a region often discussed as the “forbidden gap.” That tension has helped keep GW190521 near the center of debate, even as the broader catalog of …