Are neutrinos dark matter?

No — ordinary neutrinos are not the dark matter of the universe. They contribute only a small fraction (about 0.1–1 %) of the total dark matter mass, and the bulk is something else entirely — most likely a yet-undiscovered particle.

The idea that neutrinos might be dark matter was taken seriously through the 1980s, when their mass was unknown and they were a natural candidate. Three decades of measurements have now ruled them out as the dominant component.

Why neutrinos can’t be dark matter

Two physical arguments rule out ordinary (active) neutrinos as the bulk dark matter:

They’re too light

The total mass-energy of the universe is split roughly:

• 5 % ordinary matter (atoms, you, stars).
• 27 % dark matter (the unknown stuff that holds galaxies together).
• 68 % dark energy (the thing accelerating cosmic expansion).

For neutrinos to be dark matter, their combined mass-energy density today would need to match that 27 %. The cosmic neutrino number density (the CνB) is well-measured at about 336 neutrinos per cm³. To produce 27 % of cosmic mass-energy, each neutrino would need to weigh roughly 10 eV.

KATRIN’s current upper limit on the effective electron-neutrino mass is 0.45 eV — twenty times smaller. Cosmological measurements push the sum of all three neutrino masses below about 0.1 eV.

The masses just aren’t large enough.

They were too fast

In the early universe, neutrinos were ultra-relativistic. Even today they move at a substantial fraction of light speed when their kinetic energy exceeds their rest mass — which it usually does.

Fast-moving particles free-stream out of small structures. If neutrinos dominated dark matter, they would have prevented small galaxies and dwarf systems from forming. We observe both. So whatever dark matter is, most of it must be cold (slow-moving and non-relativistic) from the start.

Neutrinos are at best a hot dark matter component, and we know the universe doesn’t tolerate more than a small fraction of hot dark matter without conflict with observations.

What neutrinos do contribute

A small but real fraction of dark matter is neutrinos. The current best estimate of their contribution to the universe’s total energy budget:

$$\Omega_\nu \approx 0.001 \text{ to } 0.01$$

That’s a fraction of a percent of cosmic energy density — small, but not zero. Future cosmological measurements (CMB-S4) should pin down the sum $\Sigma m_\nu$ to ~30 meV precision, which fixes the neutrino contribution.

What about sterile neutrinos?

There’s a more speculative dark-matter candidate that is called a neutrino: the sterile neutrino. A hypothetical fourth-flavour neutrino that doesn’t feel the weak force could be much heavier than ordinary neutrinos — anywhere from a few keV to many GeV.

Sterile neutrinos with masses around 1–50 keV are plausible warm-dark-matter candidates. They would solve some galactic-scale issues with pure cold-dark-matter models and would decay slowly to active neutrino + photon, producing an X-ray line.

In 2014, several X-ray observations (XMM-Newton, Chandra) reported a marginal signal at 3.5 keV from galaxy clusters. The line has not been confirmed at high significance and is now considered ambiguous. Future observations (XRISM, Athena) plus direct kinematic searches (KATRIN’s TRISTAN upgrade) will settle whether keV sterile neutrinos exist.

The dark-matter candidates that remain

After ruling out ordinary neutrinos, the leading dark-matter candidates are:

• WIMPs (Weakly Interacting Massive Particles) — heavy particles from supersymmetric or other beyond-Standard-Model extensions. Direct-detection limits (XENONnT, LUX-ZEPLIN) are tightening rapidly.
• Axions — very light particles motivated by the strong-CP problem. ADMX and HAYSTAC are looking for them.
• Sterile neutrinos — the variant discussed above.
• Primordial black holes — small black holes formed in the early universe. Heavily constrained but not entirely ruled out.

None has been definitively detected. The dark-matter mystery is one of the biggest open questions in physics, and ordinary neutrinos are no longer on the candidate list.

The short answer

Ordinary neutrinos make up about 0.1–1 % of the dark matter in the universe. The bulk — about 99 % of dark matter — is something else. The leading candidates are WIMPs, axions, sterile neutrinos at the keV scale, or other exotica. None has been detected. The hunt continues.

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For more on the sterile-neutrino possibility, see Sterile neutrinos: a stubborn maybe. For how cosmology constrains the neutrino mass, see Cosmological bound on the neutrino mass.

Neutrino Times Editorial Team. (2025, June 28). Are neutrinos dark matter?. Neutrino Times. https://neutrino-times.com/articles/are-neutrinos-dark-matter/

Neutrino Times Editorial Team. "Are neutrinos dark matter?." Neutrino Times, June 28, 2025. https://neutrino-times.com/articles/are-neutrinos-dark-matter/.

Neutrino Times Editorial Team. "Are neutrinos dark matter?." Neutrino Times, 28 Jun. 2025, https://neutrino-times.com/articles/are-neutrinos-dark-matter/.

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  author       = {Neutrino Times Editorial Team},
  title        = {Are neutrinos dark matter?},
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  url          = {https://neutrino-times.com/articles/are-neutrinos-dark-matter/},
  note         = {Accessed: 2025-06-28}
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