Listed on a station in astrometrics
Interstellar matter was any type of material found in the outer space between star systems, such as interstellar dust.
Aboard USS Voyager, a subspace communication station in astrometrics listed information on interstellar matter alongside plasma wave probes, solar wind dispersion, cometary ions, prestellar nebulae, and nucleosynthesis. (VOY: "Ashes to Ashes", okudagram)