A recent study has investigated how the presence of dark matter (DM) affects the conformal properties and phase structure of dense matter in neutron stars. Using a self-consistent single-fluid framework, researchers have developed an exact decomposition of the total trace anomaly (Δtot) into its microscopic contributions. This approach allows for the analysis of dark matter's impact not only on the composition but also on the fundamental physics of these stars, which serve as natural laboratories for matter under extreme conditions.

For collider-motivated Higgs-portal dark matter benchmarks, direct contributions from Higgs, vector-mediator, and contact interactions are found to be negligible. However, heavy nonrelativistic dark matter possesses an intrinsic trace anomaly close to the nonrelativistic limit (Δχ ≈ 1/3). This implies that the dark matter rest-mass energy fraction is the dominant factor in modifying Δtot, producing a smooth upward shift of up to approximately 0.1 for a dark matter fraction Fχ of about 0.2% of the baryonic mass. In the pressureless, comoving heavy-WIMP regime, a universal dark-sector scaling governed by the mass-loading parameter λ = Fχmχ/mN, where mχ is the dark matter particle mass and mN is the nucleon mass, has been identified.

Numerical calculations demonstrate that, for a fixed λ, different combinations of Fχ and mχ exhibit overlapping responses in the trace anomaly, sound speed, and mass-radius relations for a given baryonic equation of state. In hybrid stars, dark matter leaves the coexistence pressure and chemical potential essentially unchanged, whereas first-order hadron-quark deconfinement produces sharp discontinuities in the squared sound speed (c_s²), Δtot, and Δtot - ΔB. This combined softening substantially reduces the maximum stellar mass, placing predictions in tension with observed 2 M☉ neutron stars if Fχ is around 0.2%.

These findings suggest that the trace anomaly and its density evolution provide sensitive diagnostics for distinguishing smooth dark matter-induced modifications from genuine first-order deconfinement. This opens new avenues for understanding the internal composition of neutron stars and potentially for detecting the presence of dark matter in these compact objects, which would have profound implications for astrophysics and particle physics.