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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, they would interact via quantum-mechanical effects even at the distances between them within regions of high dark-matter density such as the centers of galaxies. The current favored dark-matter model, cold dark matter (CDM), has the problem that simulations produce more small-scale variation than is observed, 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 unrealistic small-scale variations seen in simulations.
The conceived mass is orders-of-magnitude below range of masses measured by experiment or current normal means of detecting masses, and far less than current estimates for known and theorized low-mass particles such as axions and neutrinos, which are generally thought to have masses greater than 10-5 eV. Though these are the favored estimates, the minimum-mass limits that have been determined are sufficiently close (within a couple of orders of magnitude) to 10-22 eV to allow some interest in FDM theories.
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.