Compare · The Dark Universe

Fuzzy Dark Matter vs Sterile Neutrinos

Dark Matter Candidates· within family

	Fuzzy Dark Matter Frontier	Sterile Neutrinos Frontier
Proposed	2000	1994 / 1999
Key figures	Wayne Hu, Rennan Barkana, Andrei Gruzinov, Lam Hui	Scott Dodelson, Lawrence Widrow, Xiang-Dong Shi, George Fuller
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 neutrinos with masses around a few keV that don't feel the weak force at all, only gravity and a tiny mixing with the three Standard Model neutrinos. They would warm-dark-matter the universe and decay into X-rays, producing a detectable line in galaxy spectra. A tentative 3.5 keV X-ray line has been the subject of a decade-long debate.
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	Radiative decay line at E ≈ m_s / 2 in galaxy and galaxy-cluster X-ray spectra, with line strength scaling with dark-matter column density Small-scale structure suppression: altered Lyman-α forest at z ~ 3-5, fewer dwarf satellites of the Milky Way, lower-density halo cores If an X-ray decay line is detected, its energy and brightness would pin down both the sterile neutrino's mass and how strongly it mixes with ordinary neutrinos (the mixing angle θ), since the production rate scales as sin²(2θ) × (m_s)²
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	Hitomi (2017) failed to confirm the 3.5 keV line in the Perseus cluster (the satellite was destined for deeper sensitivity but lost before extended observations) Dessert et al. (2020) found the line absent in XMM blank-sky observations, arguing the original signal is inconsistent with a dark-matter origin Lyman-α forest and dwarf-galaxy structure constrain the simplest Dodelson-Widrow production, disfavoring sterile neutrinos as 100% of dark matter unless production is tuned The active-sterile mixing parameter space is heavily constrained by X-ray searches (XMM, Chandra, NuSTAR) over a wide mass range
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 3.5 keV line ambiguity: a faint X-ray glow that could be sterile neutrinos decaying has neither been confirmed nor ruled out, and researchers are waiting on sharper XRISM data to settle whether it is real or an instrument artifact.
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Fuzzy Dark Matter

2000 · Frontier

Sterile Neutrinos

1994 / 1999 · Frontier

Proposed

2000

1994 / 1999

Key figures

Wayne Hu, Rennan Barkana, Andrei Gruzinov, Lam Hui

Scott Dodelson, Lawrence Widrow, Xiang-Dong Shi, George Fuller

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 neutrinos with masses around a few keV that don't feel the weak force at all, only gravity and a tiny mixing with the three Standard Model neutrinos. They would warm-dark-matter the universe and decay into X-rays, producing a detectable line in galaxy spectra. A tentative 3.5 keV X-ray line has been the subject of a decade-long debate.

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

Radiative decay line at E ≈ m_s / 2 in galaxy and galaxy-cluster X-ray spectra, with line strength scaling with dark-matter column density
Small-scale structure suppression: altered Lyman-α forest at z ~ 3-5, fewer dwarf satellites of the Milky Way, lower-density halo cores
If an X-ray decay line is detected, its energy and brightness would pin down both the sterile neutrino's mass and how strongly it mixes with ordinary neutrinos (the mixing angle θ), since the production rate scales as sin²(2θ) × (m_s)²

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

Hitomi (2017) failed to confirm the 3.5 keV line in the Perseus cluster (the satellite was destined for deeper sensitivity but lost before extended observations)
Dessert et al. (2020) found the line absent in XMM blank-sky observations, arguing the original signal is inconsistent with a dark-matter origin
Lyman-α forest and dwarf-galaxy structure constrain the simplest Dodelson-Widrow production, disfavoring sterile neutrinos as 100% of dark matter unless production is tuned
The active-sterile mixing parameter space is heavily constrained by X-ray searches (XMM, Chandra, NuSTAR) over a wide mass range

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 3.5 keV line ambiguity: a faint X-ray glow that could be sterile neutrinos decaying has neither been confirmed nor ruled out, and researchers are waiting on sharper XRISM data to settle whether it is real or an instrument artifact.

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