At the centers of many galaxies, not only supermassive black holes lurk, but also vast clouds of invisible matter. If this matter consists of ultralight particles, it can gather into dense clumps — a kind of 'star' made of dark matter. For one candidate, the Proca field (a vector field similar to the electromagnetic one but massive), such clumps are called Proca stars.
When a small body, such as a neutron star, passes through such a Proca star, it slows down — losing energy that goes into gravitational waves. A group of physicists calculated exactly how much energy is lost. They compared two scenarios: a spherically symmetric Proca star and its ground state without symmetry. It turned out that in the ground state, energy losses are almost the same as for similar clumps made of a scalar field (another dark matter candidate). The difference does not exceed 20%.
This means that gravitational waves from such events will be louder, and future detectors might be able to hear them. So far, the calculations were done in a simplified Newtonian picture, without relativistic effects. But it is precisely in the full relativistic version that the differences between Proca stars and scalar clumps should become more noticeable. Then, from the signal, one could determine what dark matter is actually made of — vector or scalar particles.
Ultralight dark matter particles can be billions of times lighter than an electron, yet gather into clumps the size of an entire galaxy.
In Peter Watts's novel 'Blindsight', dark matter turns out to be intelligent, but here it simply condenses into stars that can be heard.