Two heavy balls orbit on the surface of a pond. Ripples spread from them: these are gravitational waves. As the balls spiral closer, the waves carry away energy, and the ripples become more frequent. Astronomers detect such ripples from pairs of supermassive black holes: they are billions of times heavier than the Sun and sit in the centers of distant galaxies.
But the pond might not contain only water. An invisible jelly could slow the balls down even more. Sometimes, ultralight dark matter plays the role of such jelly. Its particles are billions of times lighter than an electron, but they gather into giant clumps — solitons. When black holes pass through such a clump, it saps their energy, like jelly does to balls. This speeds up the merger and alters the wave pattern.
The authors of the new study used data from a pulsar network spanning many years. They compared four explanations for how black holes lose energy. The first is gravitational waves alone. The second is waves plus a simple dark matter effect, as if jelly were evenly spread across the pond. The third is a more realistic picture where dark matter clumps are scattered randomly. The fourth is no dark matter at all, but with similar drag from gas and stars around the black holes.
The result is cautious. No model won by a clear margin. But within the dark matter family, the simple version outperformed the realistic one at all tested frequencies. This means the data cannot yet distinguish dark matter from ordinary astrophysical noise, but they don't rule it out either. Future observations might let us hear how the invisible jelly alters the cosmic ripples.
Solitons of ultralight dark matter can be as large as an entire galaxy, yet consist of particles $10^{22}$ times lighter than an electron, like a fog cloud weighing less than a grain of sand.
In Fred Hoyle's novel "The Black Cloud," a sentient gas cloud slows down a star, a similar principle of dynamical friction.