Fuzzy cold dark matter

Fuzzy cold dark matter is a hypothetical form of cold dark matter proposed to solve the cuspy halo problem. It would consist of extremely light scalar particles with masses on the order of ${\displaystyle \approx 10^{-22}}$ eV; so a Compton wavelength on the order of 1 light year. Fuzzy cold dark matter halos in dwarf galaxies would manifest wave behavior on astrophysical scales, and the cusps would be avoided through the Heisenberg uncertainty principle.^[1] The wave behavior leads to interference patterns, spherical soliton cores in dark matter halo centers,^[2] and cylindrical soliton-like cores in dark matter cosmic web filaments.^[3]

Fuzzy cold dark matter is a limit of scalar field dark matter without self-interaction.^[4][5] Fuzzy cold dark matter is sometimes called wave DM, or simply fuzzy dark matter (FDM).^[6] It is governed by the Schrödinger–Poisson equation.

Fuzzy dark matter models are the simplest class of the ultralight dark matter models; the only free parameter is the particle mass. (In "ultralight dark matter models", the dark matter of a galaxy condenses into a superfluid. This requirement greatly constrains the particle mass; for example, the QCD (Peccei–Quinn) axion is considered too heavy to condense.) A second approach, where FDM is modified to have simple self-interaction, has been suggested with theories such as self-interacting fuzzy dark matter (SIFDM), repulsive DM, scalar field DM, and fluid dark matter. A third approach, called the "DM superfluid model", focuses on the empirical data for a large-scale MOND relation, and then works backwards to determine what types of complicated self-interactions would best produce such a distribution.^[6]

New research (2023) has uncovered evidence that fuzzy dark matter, specifically ultralight axions, may better fit gravitational lens data than WIMP dark matter.^[7]