At the center of attention are tiny stellar cities: ultra-faint dwarf galaxies. They resemble sparse swarms of fireflies circling within a vast invisible cloud. This cloud is a halo of ultralight dark matter. Its particles are so light that they behave not like billiard balls but like waves. When these waves overlap, they can 'rock' the stars, adding energy to them, so the stellar swarm gradually puffs up.
But this swarm has its own internal squabbles. Stars exchange gravitational nudges, like a queue where everyone impatiently pushes their neighbor. This friction, known as two-body relaxation, also heats the system. Previously, it was often overlooked, but that was a mistake: it can be more important than wave heating. Add to this the chemical composition of the stars: their metallicity. Stars rich in heavy elements (astronomers call everything heavier than helium 'metals') shine differently and hold onto each other more tightly. In simulations, they proved more resistant to disruption.
Finally, the Milky Way enters the arena. Its tidal field acts like an invisible conductor, altering the orbits of stellar swarms within the dark halo. Depending on the mass of the dark matter particles and the size of the system, heating can behave in opposite ways. When the swarm is much smaller than the characteristic wavelength of the particle, increasing the particle mass enhances heating, as if the waves start beating more frequently and precisely. This complex picture reminds us that simple recipes don't work here. Previous constraints on particle mass, derived from observations of such systems, need to be re-examined considering all three factors: internal friction, metallicity, and tides. And then the invisible conductor will sound clearer.
The de Broglie wavelength for ultralight dark matter can reach hundreds of light-years — longer than the distance to the nearest stars.
In Peter Watts's novel 'Blindsight', alien life forms use the wave properties of matter to survive in dark clouds.