A faint arc of stars 115 million light-years away, named Oyashio, may be the first stellar stream ever detected beyond our own galaxy — a gossamer thread of escaped suns trailing from a disrupted globular cluster around an ultra-diffuse galaxy. Astronomers have long known that such streams act as natural seismographs of gravitational fields, and now, for the first time, that instrument may be readable in a distant and otherwise inscrutable corner of the cosmos. If confirmed, Oyashio would not merely be a discovery but a method — a way of weighing the invisible dark matter that shapes galaxies