Compare · The Dark Universe

WIMPs vs Self-Interacting Dark Matter (SIDM)

Dark Matter Candidates· within family

	WIMPs Frontier	Self-Interacting Dark Matter (SIDM) Frontier
Proposed	1977 / 1996	2000 / 2018
Key figures	Benjamin Lee, Steven Weinberg, Gerard Jungman, Gianfranco Bertone	David Spergel, Paul Steinhardt, Sean Tulin, Hai-Bo Yu
In one sentence	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.	Dark matter that interacts with itself via some non-gravitational force, with cross sections tuned so the interactions thermalize the inner regions of dwarf galaxies (creating cores instead of cusps) but barely affect large-scale structure.
Predictions	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)	Cored density profiles in dwarf and low-surface-brightness galaxies, with core sizes correlated with halo mass and velocity dispersion Reduced central densities and rounder inner halos in galaxy clusters compared to pure CDM, but a smaller effect than in dwarfs (because clusters have higher velocity dispersion and shorter halo crossing times relative to the SIDM mean free path) Possible offsets between dark matter and galaxies in merging cluster systems, depending on the velocity dependence of the cross-section
Where it breaks	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	Baryonic physics (star formation feedback, AGN) can also produce cores within CDM, reducing the necessity of SIDM Cluster shapes and ellipticity from gravitational lensing constrain σ/m below the value needed to affect dwarf cores, requiring velocity-dependent cross-sections that some models can produce but not all Missing-satellites and too-big-to-fail problems aren't fully addressed by SIDM alone; they require additional fixes Critics view SIDM as introducing a free parameter (the cross-section) rather than proposing a specific candidate particle
Key unresolved problem	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.	The mimicry problem: ordinary effects like exploding stars and black-hole winds can carve out the same smooth galaxy centers that self-interacting dark matter (SIDM) would, so nothing observed yet clearly requires the particles to collide with each other.
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WIMPs

1977 / 1996 · Frontier

Self-Interacting Dark Matter (SIDM)

2000 / 2018 · Frontier

Proposed

1977 / 1996

2000 / 2018

Key figures

Benjamin Lee, Steven Weinberg, Gerard Jungman, Gianfranco Bertone

David Spergel, Paul Steinhardt, Sean Tulin, Hai-Bo Yu

In one sentence

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.

Dark matter that interacts with itself via some non-gravitational force, with cross sections tuned so the interactions thermalize the inner regions of dwarf galaxies (creating cores instead of cusps) but barely affect large-scale structure.

Predictions

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)

Cored density profiles in dwarf and low-surface-brightness galaxies, with core sizes correlated with halo mass and velocity dispersion
Reduced central densities and rounder inner halos in galaxy clusters compared to pure CDM, but a smaller effect than in dwarfs (because clusters have higher velocity dispersion and shorter halo crossing times relative to the SIDM mean free path)
Possible offsets between dark matter and galaxies in merging cluster systems, depending on the velocity dependence of the cross-section

Where it breaks

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

Baryonic physics (star formation feedback, AGN) can also produce cores within CDM, reducing the necessity of SIDM
Cluster shapes and ellipticity from gravitational lensing constrain σ/m below the value needed to affect dwarf cores, requiring velocity-dependent cross-sections that some models can produce but not all
Missing-satellites and too-big-to-fail problems aren't fully addressed by SIDM alone; they require additional fixes
Critics view SIDM as introducing a free parameter (the cross-section) rather than proposing a specific candidate particle

Key unresolved problem

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.

The mimicry problem: ordinary effects like exploding stars and black-hole winds can carve out the same smooth galaxy centers that self-interacting dark matter (SIDM) would, so nothing observed yet clearly requires the particles to collide with each other.

Reader vote

No votes yet