Fuzzy Dark Matter (Ultralight Axions)

Fuzzy Dark Matter's ultralight axions, with masses around 10⁻²² eV, have de Broglie wavelengths spanning entire galaxies, suppressing small-scale structure and explaining the dwarf galaxy deficit versus cold dark matter. Yet Lyman-alpha forest data from Keck Observatory tightens constraints, pushing the axion mass above 2 × 10⁻²¹ eV. This candidate is cheaper than the typical rival in particle mass, but remains less favored than #6 SIDM due to these stricter bounds, with a concrete mass limit of 5 × 10⁻²¹ eV from quasar data.