Dark matter with an excited state fits the El Gordo cluster collision best in new simulations
Simulations of the El Gordo collision, over two quadrillion solar masses, fit two-state self-interacting dark matter better than noninteracting dark matter. Phys.org's account calls the result a possible solution, not a definite one.
The Scientist · Science desk
Five actors. Some astronomers hold El Gordo as a counterexample to the standard model. Cold dark matter struggles to build it so quickly. Self-interacting dark matter could, but makes warm dark matter. A two-state model fits best in simulations. At low-end mass and speed it would not fit.
- What doesn't add up contradiction Standard model of cosmology Some astronomers hold El Gordo, seven billion light-years away, as a counterexample to it, claim 1
- What it limits constraint Cold dark matter Hard to explain how it could build a supercluster as quickly as El Gordo formed, claim 3
- What it costs cost Self-interacting dark matter Could build an early supercluster like El Gordo, but would produce plenty of warm dark matter, claim 5
- What's now possible capability Two-state dark matter model Best fit in N-body simulations of El Gordo, ahead of noninteracting and elastic-only models, claim 8
- What it limits constraint Mass and speed estimates Still uncertain; if El Gordo is at the low end of the range, the two-state model would not fit, claim 9
| Who | How | Kind | Claim |
|---|---|---|---|
| Standard model of cosmology | Some astronomers hold El Gordo, seven billion light-years away, as a counterexample to it | contradiction | 1 |
| Cold dark matter | Hard to explain how it could build a supercluster as quickly as El Gordo formed | constraint | 3 |
| Self-interacting dark matter | Could build an early supercluster like El Gordo, but would produce plenty of warm dark matter | cost | 5 |
| Two-state dark matter model | Best fit in N-body simulations of El Gordo, ahead of noninteracting and elastic-only models | capability | 8 |
| Mass and speed estimates | Still uncertain; if El Gordo is at the low end of the range, the two-state model would not fit | constraint | 9 |
What happened
- El Gordo is seven billion light-years away and unusually large for its era, and some astronomers treat it as a counterexample to the standard cosmological model.
- Ordinary self-interaction could build such a cluster early, but it would also produce plenty of warm dark matter, and observations show dark matter is mostly cold.
- The proposed fix gives dark matter a ground state and an excited state, so a collision at the right energy can shed kinetic energy and let particles cluster and cool together.
- The paper is a single-author study by R. Valdarnini, published in The Astrophysical Journal in 2026.
Why it matters
- Who has to decide decision Observers who tighten the mass and speed estimates for El Gordo can now keep or drop this model, so those measurements carry more weight than the simulation itself.
- What's now possible capability The excited state gives modelers a way to speed up clustering and cool the dark matter in the same collision, a combination that elastic-only tuning has handled with mixed results.
- What doesn't add up contradiction The cluster some astronomers call a counterexample to the standard model is here used to favor a different model. Phys.org describes a fit for this one system only, so other clusters remain untested in its account.
The design is a three-way comparison. The colliding regions of El Gordo were modeled with noninteracting dark matter, with elastic-only self-interaction, and with the two-state version, and the two-state model fit best [8]. The noninteracting run is the control, since it is the most popular view of dark matter [4]. The elastic-only run is the closer rival. Elastic self-interaction can build an early cluster, but it leaves plenty of warm dark matter [5], and tuning it has given mixed results [6]. The excited state is the one added ingredient: a collision at the right energy can remove kinetic energy from the pair [7].
The thing this doesn't tell you is the size of the margin. Phys.org's summary does not include fit statistics, so "best fit" here is a ranking. Phys.org also describes N-body simulations, while the paper's title refers to hydrodynamical ones [8][11].
The conclusion depends on two inputs that are not pinned down: El Gordo's mass and the speed of the collision [9]. If the cluster sits at the low end of that range, the two-state model would not fit the data [9]. A tighter mass or speed estimate could therefore confirm the fit or take it away.
We think the result is best read as one candidate model that passes one test, on conditions the inputs set. The motivation is real, since some astronomers already treat El Gordo as a counterexample to the standard model [1]. Phys.org's closing point is that until dark matter particles are observed directly, "all sorts of possibilities are on the table" [12].
What to watch
- Tighter estimates of El Gordo's mass and collision speed: a value at the low end of the current range would remove the two-state model's fit.
- Whether other merging clusters give the same ranking of two-state, elastic-only and noninteracting dark matter.
- Any direct observation of dark matter particles, which would replace simulation fits with a measured interaction.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence45
- Adoption
- Not scored
- Hype gap−10
- Incentives
- Not scored
- Confidence40
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
El Gordo is seven billion light-years from Earth and is extraordinarily large for that period of the universe; some astronomers hold it as a counterexample to the standard model of cosmology.
- [2]
El Gordo has two regions in the process of colliding with each other, with a combined mass of more than two quadrillion solar masses.
- [3]
It is difficult to explain how cold dark matter in an inflationary universe could generate a large supercluster as quickly as El Gordo formed.
- [4]
The most popular view is that dark matter is not self-interacting, because observations show dark matter is mostly cold.
- [5]
Self-interacting dark matter could create an early supercluster like El Gordo, but it would also produce plenty of warm dark matter.
- [6]
Tweaking the usual elastic interaction to accelerate galactic clustering without overly warming dark matter has had mixed results.
- [7]
The study considers a dark matter model with a ground state and an excited state: most interactions are elastic, but a collision at the right energy can kick a particle into the excited state, removing kinetic energy from the collision, so particles can cluster and cool at the same time.
- [8]
Based on N-body simulations of the colliding regions of El Gordo, a two-state self-interacting dark matter model with both elastic and inelastic collisions is the best fit to observations, better than both noninteracting and elastic-only interactions.
- [9]
There is still considerable uncertainty about El Gordo's exact mass and the speed at which its two regions are colliding; if the cluster is at the low end of this range, the two-state model would not fit the data.
- [10]
Phys.org's account concludes that the two-state model is a possible solution, not a definite one.
- [11]
The paper is R. Valdarnini, 'Hydrodynamical Simulations of the Merging Cluster El Gordo in a Two-state Self-interacting Dark Matter Scenario', The Astrophysical Journal (2026), DOI 10.3847/1538-4357/ae9300; Phys.org refers to 'the author' in the singular.
- [12]
Until we directly observe dark matter particles, all sorts of possibilities are on the table.
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Sources
1 independent publisher whose own reporting we read for this story.
- phys.orgA distant supercluster of galaxies points to self-interacting dark matter
1 article · October 16, 2026
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