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Sub-GeV dark matter (or light dark matter) indicates a hypothesized dark matter particle with a mass less than equivalent to a GeV, more massive than warm dark matter (WDM) but less massive than indicated by the cold dark matter (CDM) theories that have seemed most promising. A higher mass is necessary for some earlier promising theories, but lengthy efforts have failed to detect such particles, and the sub-GeV dark matter concept has regained interest over the past few years.
The term very light dark matter is occasionally used, e.g., for mass on the order a keV and less. The term ultralight dark matter is also used, e.g., for an eV or less, including fuzzy dark matter (which is an extreme example, presumed 22 dex lighter than an eV).