Particles of dark matter can behave like molasses: in one model they don't interfere with each other; in another, they gently jostle, creating viscosity. This determines how quickly giant black holes at the centers of galaxies fall into each other and merge.
The LISA space observatory will catch gravitational waves from such mergers. If about 70 signals are registered, their frequency will reveal whether dark matter was viscous. This method is reminiscent of how, by the thud of falling apples, you can tell whether they landed on grass or in syrup. Remarkably, LISA is designed much like the human auditory system: three detectors, spaced millions of kilometers apart, act like our ears, picking up the tremors of spacetime.
Fritz Zwicky suspected dark matter's existence in 1933, and Vera Rubin proved it in the 1970s: galaxies rotate as if held by an invisible framework. Today it's clear: there's five times more dark matter than ordinary matter.