Compare · The Dark Universe

Fuzzy Dark Matter vs WIMPs

Dark Matter Candidates· within family

	Fuzzy Dark Matter Frontier	WIMPs Frontier
Proposed	2000	1977 / 1996
Key figures	Wayne Hu, Rennan Barkana, Andrei Gruzinov, Lam Hui	Benjamin Lee, Steven Weinberg, Gerard Jungman, Gianfranco Bertone
In one sentence	Fuzzy Dark Matter is an ultra-light scalar (mass ~10^-22 eV) whose de Broglie wavelength reaches kpc scales, producing wave-mechanical phenomenology in galactic dynamics. The framework was introduced by Hu, Barkana, and Gruzinov in 2000 (Phys. Rev. Lett. 85, 1158) and substantially developed in the 2017 Hui-Ostriker-Tremaine-Witten paper Ultralight scalars as cosmological dark matter (Phys. Rev. D 95, 043541). Distinct from generic Axions due to its ultra-light mass and the resulting wave-mechanical effects on galactic scales.	Hypothetical particles with masses around the weak scale that interact with ordinary matter via the weak nuclear force and gravity but not light. Their thermal abundance from the early universe naturally matches the observed dark matter density. Direct-detection experiments have been searching for decades and have not seen them.
Predictions	Galactic dark-matter halos have minimum core size set by the de Broglie wavelength of the FDM particle; sub-kiloparsec cores are predicted for ~10^-22 eV particles Dwarf-galaxy cores are dominated by solitonic structures, the ground-state quantum-wave configurations of the FDM particle in a self-gravitating halo Lyman-alpha forest measurements should detect the wave-mechanical suppression of small-scale structure at masses below the constraint threshold Specific signatures in the matter power spectrum on small scales that distinguish FDM from generic cold dark matter	Nuclear recoils with characteristic energy spectrum in xenon or argon detectors at rates set by WIMP-nucleon cross-section, WIMP mass, and local halo density Gamma rays, antimatter, and neutrinos from WIMP-WIMP annihilation in the Galactic center, dwarf spheroidals, and the Sun Missing transverse momentum in LHC events with one or more visible particles (mono-jet, mono-photon, mono-Z) A leftover abundance from the early universe that naturally matches today's dark matter density, the 'WIMP miracle', set by thermal freeze-out (Ωh² ≈ 0.12 for an annihilation rate ⟨σv⟩ ≈ 3×10^-26 cm³/s)
Where it breaks	Lyman-alpha forest constraints place lower bounds on the FDM particle mass around 10^-21 eV or higher; the original ~10^-22 eV proposal is now disfavored Dwarf-galaxy observations are in tension with the FDM predictions for solitonic-core sizes; current best-fit FDM masses produce cores too large for some observed dwarfs Distinguishing FDM from generic CDM observationally requires precise measurements at very small scales; current data places constraints but does not unambiguously favor one over the other The framework requires a specific ultra-light particle mass and self-interaction structure; the deep physical motivation for these values from string compactification or alternative UV physics is not crisp	LHC has produced no evidence of weak-scale supersymmetric WIMPs (neutralinos); natural SUSY models with TeV-scale masses now require fine-tuning Direct-detection limits exclude the simplest weak-cross-section WIMPs for masses where the WIMP miracle was strongest The WIMP miracle is less compelling post-LHC: many of the natural models that gave the coincidence are now constrained or ruled out Critics argue WIMPs are becoming unfalsifiable: every null result is met with a more elaborate model variant
Key unresolved problem	The Lyman-alpha tension: the fine structure seen in distant gas clouds (the Lyman-alpha forest) now rules out the original ultra-light particle mass near 10^-22 eV, leaving only somewhat heavier, still-unconfirmed values in play.	The neutrino fog problem: detectors are now so sensitive they pick up a steady drizzle of natural neutrinos, and it is unclear whether a real WIMP signal could ever be told apart from this unavoidable background.
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Fuzzy Dark Matter

2000 · Frontier

WIMPs

1977 / 1996 · Frontier

Proposed

2000

1977 / 1996

Key figures

Wayne Hu, Rennan Barkana, Andrei Gruzinov, Lam Hui

Benjamin Lee, Steven Weinberg, Gerard Jungman, Gianfranco Bertone

In one sentence

Fuzzy Dark Matter is an ultra-light scalar (mass ~10^-22 eV) whose de Broglie wavelength reaches kpc scales, producing wave-mechanical phenomenology in galactic dynamics. The framework was introduced by Hu, Barkana, and Gruzinov in 2000 (Phys. Rev. Lett. 85, 1158) and substantially developed in the 2017 Hui-Ostriker-Tremaine-Witten paper *Ultralight scalars as cosmological dark matter* (Phys. Rev. D 95, 043541). Distinct from generic Axions due to its ultra-light mass and the resulting wave-mechanical effects on galactic scales.

Hypothetical particles with masses around the weak scale that interact with ordinary matter via the weak nuclear force and gravity but not light. Their thermal abundance from the early universe naturally matches the observed dark matter density. Direct-detection experiments have been searching for decades and have not seen them.

Predictions

Galactic dark-matter halos have minimum core size set by the de Broglie wavelength of the FDM particle; sub-kiloparsec cores are predicted for ~10^-22 eV particles
Dwarf-galaxy cores are dominated by solitonic structures, the ground-state quantum-wave configurations of the FDM particle in a self-gravitating halo
Lyman-alpha forest measurements should detect the wave-mechanical suppression of small-scale structure at masses below the constraint threshold
Specific signatures in the matter power spectrum on small scales that distinguish FDM from generic cold dark matter

Nuclear recoils with characteristic energy spectrum in xenon or argon detectors at rates set by WIMP-nucleon cross-section, WIMP mass, and local halo density
Gamma rays, antimatter, and neutrinos from WIMP-WIMP annihilation in the Galactic center, dwarf spheroidals, and the Sun
Missing transverse momentum in LHC events with one or more visible particles (mono-jet, mono-photon, mono-Z)
A leftover abundance from the early universe that naturally matches today's dark matter density, the 'WIMP miracle', set by thermal freeze-out (Ωh² ≈ 0.12 for an annihilation rate ⟨σv⟩ ≈ 3×10^-26 cm³/s)

Where it breaks

Lyman-alpha forest constraints place lower bounds on the FDM particle mass around 10^-21 eV or higher; the original ~10^-22 eV proposal is now disfavored
Dwarf-galaxy observations are in tension with the FDM predictions for solitonic-core sizes; current best-fit FDM masses produce cores too large for some observed dwarfs
Distinguishing FDM from generic CDM observationally requires precise measurements at very small scales; current data places constraints but does not unambiguously favor one over the other
The framework requires a specific ultra-light particle mass and self-interaction structure; the deep physical motivation for these values from string compactification or alternative UV physics is not crisp

LHC has produced no evidence of weak-scale supersymmetric WIMPs (neutralinos); natural SUSY models with TeV-scale masses now require fine-tuning
Direct-detection limits exclude the simplest weak-cross-section WIMPs for masses where the WIMP miracle was strongest
The WIMP miracle is less compelling post-LHC: many of the natural models that gave the coincidence are now constrained or ruled out
Critics argue WIMPs are becoming unfalsifiable: every null result is met with a more elaborate model variant

Key unresolved problem

The Lyman-alpha tension: the fine structure seen in distant gas clouds (the Lyman-alpha forest) now rules out the original ultra-light particle mass near 10^-22 eV, leaving only somewhat heavier, still-unconfirmed values in play.

The neutrino fog problem: detectors are now so sensitive they pick up a steady drizzle of natural neutrinos, and it is unclear whether a real WIMP signal could ever be told apart from this unavoidable background.

Reader vote

No votes yet