Gravitational waves pass through ordinary matter almost unimpeded. The coupling between gravitational radiation and matter is so weak that a gravitational wave can traverse the entire observable universe with negligible absorption. This transparency is why gravitational wave astronomy works — the signals from billion-year-old mergers arrive at detectors with their waveforms intact.

Dark matter might absorb them.

The mechanism is inverse bremsstrahlung: a dark matter particle scattering off another dark matter particle can absorb a gravitational wave during the collision, transferring the wave's energy into kinetic energy of the scattering pair. This is the gravitational analog of how electrons absorb photons during collisions with ions in a plasma — a process well-understood in electromagnetic theory.

The absorption depends on the dark matter mass, its self-interaction cross section, and its temperature. By measuring (or bounding) the absorption of gravitational waves passing through dark matter halos and the intergalactic medium, these parameters can be constrained. The idea is sound: use gravitational wave propagation as a probe of dark matter properties, turning the transparency of spacetime into a measurement tool by looking for where transparency fails.

The constraints turn out to be less stringent than existing methods. The gravitational wave absorption signal is weak enough that current observational limits don't improve on constraints from other probes. The dark matter is too diffuse, the coupling too feeble, the absorption too small to compete with laboratory and astrophysical bounds already in hand.

But the approach is new. Gravitational waves as dark matter probes — not through their generation (merger rates in dark matter halos) or their lensing (gravitational time delays), but through their direct absorption by the medium they traverse. A transparent universe might be less transparent than assumed, just not measurably so — yet.