Light dark matter, in astronomy and cosmology, are dark matter weakly interacting massive particles (WIMPS) candidates with masses less than 1 GeV (i.e., a mass similar to or less than a neutron or proton). These particles are heavier than warm dark matter and hot dark matter, but are lighter than the traditional forms of cold dark matter, such as Massive Compact Halo Objects (MACHOs). The Lee-Weinberg bound limits the mass of the favored dark matter candidate, WIMPs, that interact via the weak interaction to ${\displaystyle \approx 2}$ GeV. This bound arises as follows. The lower the mass of WIMPs is, the lower the annihilation cross section, which is of the order ${\displaystyle \approx m^{2}/M^{4}}$, where m is the WIMP mass and M the mass of the Z boson. This means that low mass WIMPs, which would be abundantly produced in the early universe, freeze out (i.e. stop interacting) much earlier and thus at a higher temperature, than higher mass WIMPs. This leads to a higher relic WIMP density. If the mass is lower than ${\displaystyle \sim 2}$ GeV the WIMP relic density would overclose the universe.

Some of the few loopholes allowing one to avoid the Lee-Weinberg bound without introducing new forces below the electroweak scale have been ruled out by accelerator experiments (i.e. CERN, Tevatron), and in decays of B mesons.

A viable way of building light dark matter models is thus by postulating new light bosons. This increases the annihilation cross section and reduces the coupling of dark matter particles to the Standard Model making them consistent with accelerator experiments.

Current methods to search for light dark matter particles include direct detection through electron recoil.

In recent years, light dark matter has become popular due in part to the many benefits of the theory. Sub-GeV dark matter has been used to explain the positron excess in the Galactic Center observed by INTEGRAL, excess gamma rays from the Galactic Center and extragalactic sources. It has also been suggested that light dark matter may explain a small discrepancy in the measured value of the fine structure constant in different experiments. Furthermore, the lack of dark matter signals in higher energy ranges in direct detection experiments incentivizes sub-GeV searches.

Due to the constraints placed on the mass of WIMPs in the popular freeze out model which predict WIMP masses greater than 2 GeV, the freeze out model must be altered to allow for lower mass dark matter particles.

The Lee–Weinberg limit, which restricts the mass of dark matter particles to >2 GeV may not apply in two special cases where dark matter is a scalar particle.

The first case requires that the scalar dark matter particle is coupled with a massive fermion. This model rules out dark matter particles less than 100 MeV because observations of gamma ray production do not align with theoretical predictions for particles in this mass range. This discrepancy may be resolved by requiring an asymmetry between the dark matter particles and antiparticles, as well as adding new particles.