A star made of dark matter is an invisible ocean of ultra-light particles, whose existence was first suspected by Fritz Zwicky back in the 1930s. Into this ocean plunges a neutron star — a tiny, yet unimaginably dense sphere. Friction with the dark matter, like moving through water, accelerates its fall towards the center. As it sinks, the fabric of space itself ripples, producing gravitational waves.
This process generates a "chirp" — a signal indistinguishable from the merger of two black holes. But the ending gives the impostor away: for black holes, the tone fades out smoothly, whereas for a dark star it cuts off abruptly, like a boat sinking to the bottom. Future detectors, such as LISA, will be able to spot these fakes.
A boson star could be bigger than Pluto’s orbit, yet so diaphanous that an astronaut flying through it would feel only a faint extra weight.