Self-interacting dark matter (SIDM) is a version of the dark-matter hypothesis in which dark-matter particles can scatter off one another. That extra interaction can move heat through a galaxy’s halo and soften the steep central density predicted by collisionless cold dark matter, potentially producing a broad, low-density core while leaving large-scale structure largely unchanged.
SIDM is not established as a discovery. Dwarf-galaxy rotation curves provide a motivation, while cluster density profiles, merging systems, weak-lensing shapes and rare small-scale collisions impose complementary constraints. The central question is whether one interaction law—often with a velocity-dependent cross-section—can fit all of those observations once baryonic physics and measurement systematics are modeled consistently.
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What self-interacting dark matter means
In the standard cold-dark-matter picture, dark-matter particles interact gravitationally but almost never collide. Their distribution is therefore described as effectively collisionless: particles pass through one another, and halo structure is set mainly by gravity, cosmological assembly and the effects of ordinary matter.
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SIDM adds a non-gravitational scattering process among dark-matter particles. The usual astrophysical parameter is the interaction cross-section divided by particle mass, written σ/m and measured in square centimetres per gram (cm² g⁻¹). This ratio determines how often particles scatter as they move through a halo. A larger value generally means more heat and momentum transport, but the outcome also depends on halo density, age, assembly history and the relative velocity of the particles.
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Scattering can redistribute energy from hotter outer regions toward cooler inner regions. Over time, that transport lowers the central density and can create an approximately constant-density core. On larger scales, simulations find that the abundance of subhaloes and the halo circular-velocity function can remain close to collisionless-CDM predictions, so SIDM can change inner structure without automatically destroying the model’s successes in large-scale cosmology.
Why physicists consider SIDM
Many dwarf and low-surface-brightness galaxies appear to have less sharply concentrated centres than the simplest collisionless simulations predict. Their observed rotation curves can rise gradually rather than following the steep “cusp” expected in a dark-matter-only calculation. SIDM offers a direct mechanism for reducing the central density instead of relying entirely on uncertain astrophysical feedback.
The motivation is not a single uncontested measurement. Stellar feedback, gas inflows and outflows, uncertain stellar masses, triaxial haloes and non-circular motions can all alter an inferred rotation curve. Different halo profiles can fit the same sparse kinematic data. SIDM is therefore best viewed as a physical explanation that must be tested against multiple systems, not as a conclusion drawn from dwarf galaxies alone.
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How scattering changes a halo
From collisions to a core
In a dense central region, dark-matter particles have a higher probability of scattering. Those collisions exchange energy and tend to make the inner velocity distribution more thermalized. The result is often a shallower density profile with a core whose size depends on σ/m, halo mass, concentration and the time available for evolution.
Very strong interactions do not simply produce an ever-larger stable core. Once heat conduction carries energy outward, the inner halo can lose pressure support and contract in a process known as gravothermal core collapse. Whether collapse occurs within a Hubble time depends on the interaction strength, its velocity dependence, the halo’s assembly history and baryonic gravitational potential.
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Why velocity dependence matters
Dwarf galaxies, galaxy groups and clusters have very different characteristic dark-matter velocities. A constant cross-section that affects dwarfs strongly may over-soften cluster cores. Particle models with a light mediator or another velocity-dependent interaction can make scattering efficient at dwarf velocities and weaker at cluster velocities. That possibility is why constraints must be compared across several mass and velocity scales rather than quoted as one universal number.
What simulations say about useful interaction strengths
A 2013 cosmological-simulation study found that an interaction scale near 0.1 cm² g⁻¹ could reproduce reported core sizes and central densities across dwarf galaxies, low-surface-brightness galaxies and clusters in its modeled sample. The same work found that a 1 cm² g⁻¹ model produced cores that were too diffuse for some clusters, low-surface-brightness galaxies and dwarf spheroidals. These are simulation-dependent results, not a single experimentally measured value.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitches| Interaction scale or limit | What the study or review reported | How to interpret it |
|---|---|---|
| About 0.1 cm² g⁻¹ | Matched reported core sizes and central densities across several halo classes in a 2013 cosmological simulation. | A benchmark capable of producing useful cores in that model, not a universal best fit. |
| 1 cm² g⁻¹ | Made some simulated cluster, low-surface-brightness and dwarf-spheroidal cores too low-density. | Illustrates why larger, velocity-independent interactions can be over-efficient. |
| About 0.02 cm² g⁻¹ | An earlier halo-shape analysis discussed this as a limit. | Later treatment of halo evolution and shape allowed at least 0.1 cm² g⁻¹ in the same line of work. |
| σ/mDM ≲ 0.1 cm² g⁻¹ | The Particle Data Group review summarizes a preference for this approximate upper scale from cluster central densities. | A cluster-based constraint that depends on the modeled halo population and baryonic physics. |
These numbers should not be combined as if they were measurements of one parameter from one experiment. Each comes from a particular observable, simulation setup or review of several analyses.
Evidence from dwarf and low-surface-brightness galaxies
Why these galaxies are a natural test
Dwarf galaxies have low characteristic velocities and relatively dark-matter-dominated mass budgets. If self-scattering is velocity dependent, this is where its effect can be strongest. A population of cored or otherwise diverse inner profiles would be qualitatively consistent with SIDM, especially if the diversity is difficult to obtain from identical collisionless haloes.
What prevents a clean detection
Rotation curves are not direct density measurements. Inclined discs, bars, pressure support, gas motions and limited spatial resolution can bias the inferred inner slope. Stellar-mass-to-light ratios are uncertain, and supernova-driven gas motions can transfer energy to dark matter and create cores in a collisionless model. A persuasive SIDM test therefore needs resolved kinematics, realistic baryonic simulations and a population-level comparison rather than a fit to one galaxy.
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Cluster density profiles: the high-velocity constraint
Clusters contain much more mass and have higher internal velocities than dwarfs. Their central density profiles therefore test whether an interaction remains efficient in a regime where a constant cross-section could produce excessive heat transport. Analyses using X-ray and lensing information generally disfavor very strong, velocity-independent scattering. The Particle Data Group’s review summarizes cluster-core evidence as favouring an interaction at or below roughly 0.1 cm² g⁻¹, with the precise bound varying by analysis.
The X-COP program combines observed cluster density profiles with BAHAMAS-SIDM hydrodynamical simulations to tighten this type of constraint. Such work matters because gas pressure, stellar mass and the central galaxy’s potential all influence the measured profile. A limit derived from a dark-matter-only halo cannot automatically be applied to a real cluster.
Does the Bullet Cluster rule out SIDM?
No. The Bullet Cluster is an important constraint, but it does not eliminate every self-interacting model.
During a cluster merger, galaxies pass through one another with relatively little direct interaction, while gas experiences ram pressure and forms a bright shock. If dark matter scattered often, its mass distribution could lag behind the galaxies or lose material. Early interpretations therefore sought a stringent upper bound on σ/m from the apparent alignment of dark matter and galaxies.
Later work showed that the numerical value of the bound is sensitive to how the positions of the gas, galaxies and dark-matter peaks are defined and modeled. Robertson and collaborators emphasized that the method used to measure those positions can have a larger impact on the derived constraint than is often acknowledged. Hydrodynamical simulations designed to match the observed merger can substantially weaken simplistic limits.
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The practical lesson is not that the Bullet Cluster is uninformative. It is that one merger at one relative velocity cannot determine the full velocity dependence of an interaction. New merger simulations argue for a population containing clusters, groups and galaxies with different velocities so that a model can be tested across regimes.
Halo shapes and weak lensing
Frequent self-scattering tends to make the inner halo rounder. In principle, measuring halo ellipticity could therefore distinguish SIDM from collisionless CDM. In practice, observations usually provide a projected two-dimensional shape, while the relevant physical change occurs in a three-dimensional halo. Triaxial outer material, line-of-sight structure and the orientation of the halo can mask or imitate the effect.
A 2013 study revised an earlier approximate 0.02 cm² g⁻¹ shape limit upward and found that at least 0.1 cm² g⁻¹ could be allowed after improving the treatment of halo evolution. A 2023 mock weak-lensing analysis went further: although the simulated three-dimensional shapes differed between CDM and SIDM, the distributions of weak-lensing-inferred projected cluster shapes were almost indistinguishable. Halo shape alone is therefore not a clean discriminator.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Small-scale collisions and “bullet-dwarf” systems
Rare collisions between small haloes could provide a complementary signature. If two dwarf-scale systems collide nearly head-on at high speed, self-scattering might separate dark matter from galaxies and leave a trail of dark-matter-poor remnants. A 2022 Nature analysis searched for such events in the IllustrisTNG simulation and reported 248 candidate head-on, high-velocity collisions in a modeled volume of 100³ Mpc³, with about eight within 20 Mpc in that analysis.
These counts describe simulated progenitors, not an observed census. Turning the idea into evidence requires identifying the progenitor systems, reconstructing the collision geometry, measuring the dark-matter content of the remnants and modeling their gas and star-formation histories. The rarity of the events makes selection effects especially important, but their distinct geometry could provide information that ordinary rotation curves cannot.
How the evidence compares
| Observable | Halo scale | What SIDM could change | Main limitation |
|---|---|---|---|
| Resolved rotation curves | Dwarf and low-surface-brightness galaxies | Lower central density and approximately constant-density cores | Baryonic feedback, non-circular motions and stellar-mass uncertainty |
| Cluster X-ray and lensing profiles | Groups and clusters | Core size and central concentration | Hydrostatic assumptions, gas physics and model dependence |
| Merger offsets | Cluster and group collisions | Dark-matter lag or stripping relative to galaxies | Peak-finding choices and limited velocity sampling |
| Projected halo shapes | Mostly clusters | Rounder inner mass distribution | Projection and triaxial outer structure |
| Bullet-dwarf remnants | Dwarf-scale collisions | Dark-matter-poor galaxies or trails | Rare events, progenitor identification and baryonic histories |
The major challenges
Baryonic degeneracy
Ordinary matter can reshape a halo through supernova explosions, gas inflows, star formation and the changing gravitational potential of the central galaxy. Any claim that a core proves self-interaction must show that a baryon-inclusive collisionless model cannot explain the same data. Conversely, a simulation that omits realistic gas physics may overstate the need for SIDM.
Inference and projection systematics
Lensing measures projected mass, not a three-dimensional density profile. X-ray analyses infer total mass partly from assumptions about gas pressure and equilibrium. Merger studies depend on how peaks are located, and galaxy kinematics depend on tracer selection. The Bullet Cluster, halo-shape work and weak-lensing forecasts all demonstrate that analysis methodology can move the inferred cross-section.
Core collapse and long-term evolution
A model that forms a core today may not preserve it indefinitely. Conductive heat loss can eventually reverse the evolution and make the centre contract. Predicting that transition requires following halo assembly and baryonic growth, not merely assigning a cross-section to an isolated, static halo.
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Astrophysical observations constrain scattering rates and their velocity dependence more directly than they constrain the dark-matter particle’s mass or the mediator responsible for the force. Several particle models can produce similar halo behavior. Current data therefore do not identify one unique microscopic theory.
What would count as convincing evidence?
A strong case for SIDM would require several independent observations to favour the same interaction law after systematic uncertainties are included. The most useful program has five parts:
- Measure multiple velocity regimes. Compare dwarfs, galaxies, groups and clusters rather than extrapolating one cross-section across all masses.
- Model baryons explicitly. Use matched hydrodynamical simulations with feedback, gas cooling, stellar mass and central galaxies treated consistently.
- Combine observables. Test rotation curves, cluster density profiles, merger offsets, lensing and rare collision remnants together.
- Separate detections from limits. A preference for a core, an upper bound from a merger and a non-detection in lensing answer different statistical questions.
- Check time evolution. Verify that the proposed cores survive halo assembly and do not enter gravothermal collapse too early.
Progress will come from joint analyses that span dwarf, galaxy, group and cluster velocities. If one velocity-dependent model explains central densities without violating merger and lensing constraints, while also surviving baryonic alternatives, the case will be substantially stronger than any single-galaxy result.
Bottom line on SIDM
Self-interacting dark matter is a physically motivated extension of collisionless cold dark matter. Moderate scattering can lower inner halo densities and create cores while preserving much of the large-scale clustering behavior that made CDM successful. Dwarf galaxies motivate the idea; cluster cores, mergers, halo shapes and small-scale collision remnants test its limits from different directions.
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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →The evidence remains suggestive rather than conclusive. The Bullet Cluster does not rule out SIDM, halo shape is weakened as a standalone test by projection, and the preferred interaction strength depends on velocity, baryonic physics and inference methodology. The decisive test is a consistent, multi-scale explanation—not a single headline number.
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