A binary star system orbiting through empty space conserves its orbital elements. The semi-major axis stays constant. The eccentricity stays constant. Nothing touches the orbit from outside.
A binary star system orbiting through dark matter does not. Esser (arXiv:2602.20245) shows that dynamical friction — the gravitational wake that dark matter creates behind a moving mass — acts differently on each component of a binary. The center-of-mass decelerates, but the internal orbit also evolves. The semi-major axis shrinks. The eccentricity oscillates. The binary is slowly digested by its invisible medium.
The timescales separate cleanly. Wide binaries (large separation relative to the gravitational influence radius) feel the friction on each star independently — each star drags its own dark matter wake. Close binaries feel the friction on the pair as a unit — the dark matter cannot resolve the individual stars. The transition between these regimes depends on the ratio of orbital separation to the gravitational focusing length, which depends on the dark matter density and velocity dispersion.
In ultra-faint dwarf galaxies — the most dark-matter-dominated systems known, with mass-to-light ratios exceeding 100 — the effect is measurable. Some binaries undergo significant eccentricity oscillations on timescales comparable to the age of the universe. The population of binaries in these galaxies carries an imprint of the dark matter environment they have orbited through.
The deeper point: binaries become instruments. Their orbital parameters encode information about the medium they cannot see. A binary whose eccentricity has oscillated tells you something about the dark matter density it traversed. A population of binaries with systematically altered semi-major axes tells you about the velocity dispersion of the dark matter halo. The gravitational wake that slows the binary also writes its properties into the binary's orbit.
The medium that cannot be seen can be read through its effects on things that can.