Dark Matter-Admixed Neutron Stars

• Dark matter-admixed neutron stars are compact objects where neutron matter coexists with dark matter, resulting in modified mass-radius relations and observable signatures.
• Modeling approaches include two-fluid, additive one-fluid, and chemically equilibrated frameworks that elucidate the impact of dark matter on hydrostatic structure and stellar dynamics.
• Observational constraints from gravitational waves and mass-radius measurements highlight that dark matter influences tidal deformability and oscillation frequencies, though model uncertainties remain.

Dark matter-admixed neutron stars, commonly abbreviated DMANSs or DANS, are compact stars in which ordinary neutron-star matter coexists with a dark component. In the literature, they are modeled either as genuinely two-fluid relativistic stars, with ordinary matter and dark matter coupled only through gravity, or as effectively single-fluid objects under stronger assumptions such as additive thermodynamics or chemical equilibrium between visible and dark sectors. Across these formulations, the presence of dark matter can modify the mass-radius relation, maximum mass, tidal deformability, moments of inertia, oscillation spectra, internal composition, and cooling thresholds, but the sign and magnitude of those changes depend strongly on the dark equation of state, particle mass, self-interaction strength, and spatial morphology of the dark component (Leung et al., 2011, Li et al., 2012, Shirke et al., 23 Jun 2025, Santos et al., 26 Aug 2025).

1. Conceptual frameworks and terminology

Across the current literature, three recurrent modeling classes appear. In the most common class, ordinary matter and dark matter are treated as separate perfect fluids with separate hydrostatic equations and, in general, separate radii $R_{\rm BM}$ and $R_{\rm DM}$. A second class uses a one-fluid additive prescription, in which the total pressure and total energy density are taken as sums of baryonic and dark contributions. A third class treats the star as an effectively single fluid because the dark component is assumed to be in chemical equilibrium with neutrons and to follow the hadronic density profile (Leung et al., 2011, Li et al., 2012, Shirke et al., 23 Jun 2025).

Modeling class	Defining assumption	Representative works
Two-fluid relativistic star	Separate BM and DM fluids, gravitational coupling only	(Leung et al., 2011, Kain, 2021, Santos et al., 26 Aug 2025)
Additive one-fluid model	$P=P_N+P_\chi$, $\mathcal{E}=\mathcal{E}_N+\mathcal{E}_\chi$	(Li et al., 2012, Lopes et al., 2024)
Chemically equilibrated mixed fluid	$\mu_N=\mu_\chi$; no separate DM TOV sector	(Shirke et al., 23 Jun 2025)

A further standard distinction is morphological. In two-fluid models, one speaks of a DM core when $R_{\rm DM}<R_{\rm BM}$, a DM halo when $R_{\rm DM}>R_{\rm BM}$, and a coextensive distribution when $R_{\rm DM}=R_{\rm BM}$. These are not merely descriptive labels: they determine which radius enters electromagnetic observables, which radius enters tidal response, and whether the dark component tends to increase or decrease compactness-related quantities (Santos et al., 26 Aug 2025, karan et al., 4 Jul 2026).

The early two-fluid study of Leung, Chu, and Lin identified a new class of stable compact stars consisting of a small normal-matter core with radius of a few km embedded in a ten-kilometer-sized dark-matter halo. In one representative DM-dominated configuration, the visible normal-matter radius was $R_{\rm NM}=0.56\,R$, while the mass enclosed inside that visible core was $0.72\,M$, implying a larger surface redshift than in an ordinary neutron star with the same total $R_{\rm DM}$0 and $R_{\rm DM}$1 (Leung et al., 2011).

2. Equations of state and dark-sector microphysics

The ordinary-matter sector has been modeled with a wide range of nuclear equations of state. These include APR and SLy-like descriptions, relativistic mean-field and quantum hadrodynamics models, the microscopic Brueckner-Hartree-Fock framework with hyperons, and a state-of-the-art BL equation of state for $R_{\rm DM}$2-stable nuclear matter obtained in Brueckner-Hartree-Fock using two-body and three-body interactions derived from chiral effective field theory. Some recent work goes further and samples the hadronic EOS agnostically, anchoring it at low density by chiral EFT and at high density by perturbative QCD through a speed-of-sound construction (Scordino et al., 2024, Shirke et al., 23 Jun 2025, karan et al., 4 Jul 2026).

The dark sector is even more heterogeneous. Representative choices include ideal relativistic fermion gases, self-interacting fermions parameterized by an interaction scale $R_{\rm DM}$3, quartic self-interacting bosonic condensates, neutralino-like Higgs-portal fermions, effective dark equations of state inferred from galaxy rotation curves, and strongly interacting confining dark matter based on one-flavor $R_{\rm DM}$4-QCD with lattice input (Li et al., 2012, Santos et al., 26 Aug 2025, Lopes et al., 2024, Rezaei, 2016, Dengler et al., 25 Mar 2025).

For scalar-field dark matter at zero temperature in a Bose-Einstein-condensed phase, one representative EOS used in gravitational-wave inference is

$R_{\rm DM}$5

where $R_{\rm DM}$6 is the particle mass and $R_{\rm DM}$7 the quartic self-coupling. For ideal fermionic dark matter in a realistic two-fluid study, the EOS is written parametrically as

$R_{\rm DM}$8

with $R_{\rm DM}$9 and $P=P_N+P_\chi$0 for spin-$P=P_N+P_\chi$1 particles (Santos et al., 26 Aug 2025, Scordino et al., 2024).

Despite this diversity, one modeling assumption recurs: in most two-fluid studies, ordinary matter and dark matter are taken to interact only gravitationally. Notable exceptions exist. In the Higgs-portal single-fluid models used for some ultracompact-star and radial-oscillation studies, the dark fermion is heavy, the coupling is mediated by the Standard Model Higgs, and the DM content is parameterized phenomenologically by a dark Fermi momentum $P=P_N+P_\chi$2 or $P=P_N+P_\chi$3 rather than derived from a capture history (Lopes et al., 2024, Routray et al., 2022). In the neutron-dark-decay scenario, by contrast, the dark fermion abundance is fixed by chemical equilibrium $P=P_N+P_\chi$4, and the dark density profile follows the hadronic one closely enough that the star is treated as an effectively single fluid (Shirke et al., 23 Jun 2025).

3. Hydrostatic structure, mass fractions, and core-halo morphology

In the two-fluid formalism, the total stress tensor is the sum of the ordinary and dark perfect-fluid tensors, and each sector is separately conserved. A standard form of the hydrostatic equations is

$P=P_N+P_\chi$5

with

$P=P_N+P_\chi$6

Integration starts from chosen central energy densities for the two fluids and proceeds outward until one pressure vanishes, then until the second vanishes, thereby defining $P=P_N+P_\chi$7 and $P=P_N+P_\chi$8 separately (Santos et al., 26 Aug 2025).

Different papers adopt different DM-fraction variables. In gravitational-wave parameter estimation one often uses

$P=P_N+P_\chi$9

with $\mathcal{E}=\mathcal{E}_N+\mathcal{E}_\chi$0. In the chiral-EOS and rapid-rotation literature the notation

$\mathcal{E}=\mathcal{E}_N+\mathcal{E}_\chi$1

is common, whereas in the chemically equilibrated neutron-dark-decay model the global fraction is defined energetically,

$\mathcal{E}=\mathcal{E}_N+\mathcal{E}_\chi$2

These are not interchangeable without care, because they refer to different model classes and different effective fluids (Santos et al., 26 Aug 2025, Scordino et al., 2024, Shirke et al., 23 Jun 2025, Cipriani et al., 25 Feb 2025).

Morphology is controlled primarily by the dark EOS stiffness and the dark particle mass. In the agnostic two-fluid framework, light fermionic DM tends to form extended halos, while heavy DM tends to form compact cores; specifically, $\mathcal{E}=\mathcal{E}_N+\mathcal{E}_\chi$3 yields only halo configurations and $\mathcal{E}=\mathcal{E}_N+\mathcal{E}_\chi$4 yields only core configurations in the sampled model space. The same core-halo dichotomy appears in particle-specific studies: $\mathcal{E}=\mathcal{E}_N+\mathcal{E}_\chi$5 typically produces a compact core, whereas $\mathcal{E}=\mathcal{E}_N+\mathcal{E}_\chi$6 produces a halo (karan et al., 4 Jul 2026, Scordino et al., 2024).

The one-fluid additive literature uses a different language but often reaches an analogous phenomenology. When heavy, weakly pressurized dark matter is added to the total EOS, the resulting object can become so compact that the authors themselves remark that it looks “more like DM-stars rather than NSs.” Conversely, when very light or strongly pressure-supported dark matter dominates the structure, ordinary distinctions between nucleonic and hyperonic baryonic sectors can become negligible in the mass-radius sequence (Li et al., 2012).

4. Static properties: mass-radius relations, maximum mass, and composition

No single monotonic rule covers all DMANS models. In the additive one-fluid analysis of non-self-annihilating self-interacting fermions, dark matter generally softens the EOS more strongly than hyperons and can reduce the maximum mass severely. For the interpretation of PSR J1614-2230 as a hyperon star within that framework, compatibility with the measured $\mathcal{E}=\mathcal{E}_N+\mathcal{E}_\chi$7 requires

$\mathcal{E}=\mathcal{E}_N+\mathcal{E}_\chi$8

for strongly interacting DM and

$\mathcal{E}=\mathcal{E}_N+\mathcal{E}_\chi$9

for weakly interacting DM. The same work also showed that an ambient DM halo around the star would contribute only about $\mu_N=\mu_\chi$0 to the inferred mass, so the measured mass must be attributed to the compact object itself rather than to ordinary Galactic halo material (Li et al., 2012).

In the realistic two-fluid study based on the BL chiral-BHF EOS, the sign of the effect changes with $\mu_N=\mu_\chi$1. For $\mu_N=\mu_\chi$2, the stellar gravitational maximum mass decreases as the DM fraction increases, whereas for $\mu_N=\mu_\chi$3, $\mu_N=\mu_\chi$4 increases relative to ordinary neutron stars. For the $\mu_N=\mu_\chi$5 case, the paper gives

$\mu_N=\mu_\chi$6

and finds that the direct-URCA threshold mass drops from

$\mu_N=\mu_\chi$7

to $\mu_N=\mu_\chi$8 and $\mu_N=\mu_\chi$9 at the same two fractions. In addition, for a $R_{\rm DM}<R_{\rm BM}$0 star, the observable radius $R_{\rm DM}<R_{\rm BM}$1 decreases by about $R_{\rm DM}<R_{\rm BM}$2, or about $R_{\rm DM}<R_{\rm BM}$3, at $R_{\rm DM}<R_{\rm BM}$4 (Scordino et al., 2024).

The agnostic two-fluid study reaches an important negative conclusion: dark matter does not generically compactify the star. Light halo-forming DM raises the effective radius relevant for tides and therefore increases $R_{\rm DM}<R_{\rm BM}$5, whereas heavy core-forming DM lowers it. This shifts the dominant observational constraint from gravitational-wave tidal deformability in the light-halo regime to NICER mass-radius measurements in the heavy-core regime (karan et al., 4 Jul 2026).

The same theme appears in candidate-object modeling. In the heavy, weakly interacting neutralino-like one-fluid construction proposed for XTE J1814-338, reproducing

$R_{\rm DM}<R_{\rm BM}$6

requires a DM Fermi momentum $R_{\rm DM}<R_{\rm BM}$7, yielding

$R_{\rm DM}<R_{\rm BM}$8

The same paper stresses, however, that such unusually small-radius stars are not unique evidence for dark matter admixture, since hybrid stars, strange quark stars, and other exotic interpretations remain open (Lopes et al., 2024).

5. Oscillations and stability

Radial stability has been studied most rigorously in the two-fluid framework. A broad analysis using a multi-fluid TOV system with either free-fermion or mirror dark matter showed that the stability boundary can be located both by solving the harmonic radial perturbation problem and by a static turning-point/tangency criterion; the two methods agree. One noteworthy result is that stable two-fluid stars can exist in regions where both central pressures exceed the corresponding one-fluid critical pressures, so the stability landscape is not reducible to ordinary neutron-star intuition (Kain, 2021).

In the single-fluid Higgs-portal model, dark matter softens the EOS but raises radial oscillation frequencies because the resulting stars are more compact. For a canonical $R_{\rm DM}<R_{\rm BM}$9 star, the fundamental radial frequency increases from

$R_{\rm DM}>R_{\rm BM}$0

at $R_{\rm DM}>R_{\rm BM}$1 to

$R_{\rm DM}>R_{\rm BM}$2

at $R_{\rm DM}>R_{\rm BM}$3 and

$R_{\rm DM}>R_{\rm BM}$4

at $R_{\rm DM}>R_{\rm BM}$5. In the same sequence, the critical central density shifts from

$R_{\rm DM}>R_{\rm BM}$6

to $R_{\rm DM}>R_{\rm BM}$7 and $R_{\rm DM}>R_{\rm BM}$8, while the usual criterion $R_{\rm DM}>R_{\rm BM}$9 remains valid along the stable branch (Routray et al., 2022).

A different oscillation signature arises in the chemically equilibrated neutron-dark-decay scenario, where the star is treated as compositionally stratified single-fluid matter. There the buoyancy enhancement produced by the dark component raises the $R_{\rm DM}=R_{\rm BM}$0-mode spectrum, and the shift depends predominantly on the global dark fraction rather than on hadronic effective-mass variations. The resulting quasi-universal fits are

$R_{\rm DM}=R_{\rm BM}$1

The same study argues that observing $R_{\rm DM}=R_{\rm BM}$2 frequencies as high as $R_{\rm DM}=R_{\rm BM}$3 Hz for $R_{\rm DM}=R_{\rm BM}$4 would be a “tell-tale signature” of dark matter, because ordinary hadronic stars or threshold-driven hyperon/quark phases would not generally reach such high low-mass frequencies (Shirke et al., 23 Jun 2025).

Taken together, these results suggest that DMANS stability and seismology are highly sensitive to which degree of freedom is dynamical: two-fluid radial modes probe coupled hydrostatics, whereas the neutron-dark-decay scenario probes composition-gradient buoyancy in an effectively single fluid.

6. Rotation, tides, binaries, and waveform modeling

Rotation exposes a conceptual asymmetry between electromagnetic and gravitational observables. In a slow-rotation two-fluid treatment, the star possesses separate ordinary-matter and dark-matter angular momenta and moments of inertia, while the total exterior spacetime and quadrupole are sourced by both fluids. Because electromagnetic observations directly probe ordinary matter rather than dark matter, the standard single-fluid I-Love-Q relations can deviate significantly in DM-admixed stars once the sector actually measured is distinguished from the sector that sources the external field (Cronin et al., 2023).

In the rapid-rotation regime, a fully relativistic two-fluid extension of the RNS framework shows that independent BM and DM rotation changes the equilibrium problem qualitatively. For a $R_{\rm DM}=R_{\rm BM}$5 bosonic DM core, allowing the DM itself to rotate raises the maximum mass by about $R_{\rm DM}=R_{\rm BM}$6 relative to the nonrotating-DM case, whereas in a nominal halo case the corresponding change is a decrease of about $R_{\rm DM}=R_{\rm BM}$7. The same study finds much stronger deformation for counter-rotating DM than for co-rotating DM: in representative $R_{\rm DM}=R_{\rm BM}$8, $R_{\rm DM}=R_{\rm BM}$9 core configurations,

$R_{\rm NM}=0.56\,R$0

for co-rotation and

$R_{\rm NM}=0.56\,R$1

for counter-rotation. Rapid rotation can also blur the static core-halo distinction, producing mixed geometries in which BM and DM alternate in radial dominance between pole and equator (Cipriani et al., 25 Feb 2025).

Binary dynamics amplify these structural differences. The first consistent quasi-equilibrium initial data for DM-admixed binaries showed that DM-core stars are less deformable than ordinary neutron stars of the same total rest mass, whereas DM-halo stars can undergo large halo deformation and approach mass shedding much earlier. In a representative halo case, the DM surfaces touched and mass shedding was inferred to begin between $R_{\rm NM}=0.56\,R$2 and $R_{\rm NM}=0.56\,R$3 separation, while in a representative DM-core case the baryonic matter approached mass shedding only at much smaller separations (Rüter et al., 2023).

For gravitational waves, the central observable is the dimensionless tidal deformability,

$R_{\rm NM}=0.56\,R$4

Recent infrastructure work has translated this stellar physics into waveform-generation tools. The proposed package Darksuite constructs a bank of two-fluid TOV solutions, tabulates $R_{\rm NM}=0.56\,R$5, interpolates them with linear splines, and injects the resulting $R_{\rm NM}=0.56\,R$6 values into LALSuite tidal phase corrections such as the NRTidal family, thereby enabling waveform generation for binaries containing one or more DM-admixed stars (Santos et al., 26 Aug 2025, Anh et al., 30 Jul 2025).

7. Observational constraints, candidate objects, and unresolved issues

The first direct attempt to constrain DMANSs with real gravitational-wave data reanalyzed GW230529, GW200115, GW200105, and GW190814 as NS-BH systems. For GW200105, the strongest bound among the standard events was obtained: $R_{\rm NM}=0.56\,R$7 with SLy4, and

$R_{\rm NM}=0.56\,R$8

with APR4. GW230529 and GW200115 admitted much weaker bounds but still favored DM-core configurations. GW190814 was exceptional: if one insists on soft SLy4 or APR4 hadronic EOSs, the secondary can be interpreted as a DM-halo star with

$R_{\rm NM}=0.56\,R$9

or

$0.72\,M$0

whereas with the stiffer MPA1 EOS the same event moves to a DM-core-like regime with

$0.72\,M$1

This is a clear EOS-DM degeneracy rather than evidence for a unique halo interpretation (Santos et al., 26 Aug 2025).

A more model-independent multimessenger analysis with agnostic hadronic and dark EOSs finds that current data at $0.72\,M$2 constrain the DM fraction to

$0.72\,M$3

for light dark matter. In that framework, GW170817 tidal deformability dominates the light, halo-dominated regime, whereas NICER mass-radius data dominate the heavy, core-dominated regime. The same study argues that neutron stars with similar masses but very different tidal deformabilities could be a smoking-gun signature of dark matter in neutron stars (karan et al., 4 Jul 2026).

Candidate-object interpretations remain conditional. The heavy neutralino-like model for XTE J1814-338 reproduces the reported ultracompact $0.72\,M$4-$0.72\,M$5 point only with a relatively large internal DM Fermi momentum, while HESS J1731-347 is modeled with a smaller admixture. Yet that same work states explicitly that small-radius compact stars are not definitive evidence for dark matter admixture because bosonic DM cores, hybrid stars with strong hadron-to-quark phase transitions, strange quark stars, and observational systematic effects remain plausible alternatives (Lopes et al., 2024).

Several misconceptions can therefore be excluded. Dark matter is not a generic cure for the hyperon puzzle; in the additive fermion model it usually worsens the softening problem (Li et al., 2012). Dark matter does not generically compactify neutron stars; light halos can instead raise $0.72\,M$6 (karan et al., 4 Jul 2026). And conclusions from one model class do not automatically transfer to another: a two-fluid halo/core system, a one-fluid additive EOS, and a chemically equilibrated neutron-dark-decay star probe distinct physical regimes (Shirke et al., 23 Jun 2025).

The principal open issue is model dependence. Many studies parameterize the dark content phenomenologically rather than deriving it from capture, formation, or chemical kinetics; many assume gravity-only BM-DM coupling; many ignore rotation, temperature, magnetic fields, or nongravitational interactions; and observational bounds remain entangled with the uncertainty of the ordinary high-density EOS. A plausible implication is that future progress will depend less on any single “DMANS signature” than on combining multimessenger observables—mass, radius, tidal deformability, moment of inertia, and oscillation spectra—within frameworks that track dark morphology and visible-sector EOS uncertainty simultaneously.