Fuzzy dark matter (FDM or fuzzy cold dark matter) is a model of dark matter presuming it consists of particles with rest mass on the order of 10-22eV. This quality gives it a very long Compton wavelength, on the order of a light-year, which means that in addition to interacting with each other and other matter via gravity, it would interact via quantum-mechanical effects at a considerable distance. A favored model, cold dark matter (CDM) has the problem that simulations produce too much small-scale variation, and FDM has the potential to match CDM's successes without suffering from this problem, i.e., it has a smoothing effect at just the scale to smooth out the small-scale variations seen in simulations.
The conceived mass is orders-of-magnitude below range of masses measured by experiment and current normal means of detecting masses, and far less than the estimates for established low-mass particles such as axions and neutrinos, which are thought to have masses greater than 10-5 eV. Studies have posited constraints on the masses that are possible based on various observations and the masses presumed by current FDM theories generally fall within a couple of orders of magnitude of 10-22 eV.
Dark matter particles with such a low mass would be more wave-like and would be best described by a quantum field theory (QFT). The terms wave dark matter and scalar field dark matter have also been used for the concept.