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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 1 GeV, more massive than warm dark matter (WDM) but less massive than indicated by the cold dark matter (CDM) theories. For considerable time, these higher-mass theories have seemed most promising, but lengthy efforts have failed to detect such high-mass particles, and possibility of sub-GeV dark matter has regained some 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).