Dark matter admixed neutron stars (DANSs) serve as a specific astrophysical laboratory for probing the features of dark matter (DM) and have emerged as a promising candidate for interpreting recent astrophysical observations (e.g., by NICER and LIGO/Virgo). Accurately constraining the internal DM content of DANSs is therefore of critical importance. In this work, we construct the equations of state (EOS) for DANS matter by employing twelve nuclear matter (NM) models within the covariant density functional (CDF) theory and a self-interacting fermionic model for DM. Using these EOSs as input, we solve the two-fluid Tolman-Oppenheimer-Volkoff (TOV) equations to systematically investigate the influence of DM on the global properties of neutron stars (NSs). By incorporating recent observational constraints on NS properties, the maximum DM mass fraction $f_{\chi }^{\max }$ in DANSs is determined for each NM EOS model. Our analysis reveals a strong linear correlation between $f_{\chi }^{\max }$ and the maximum mass of a pure NS, $M_{\mathrm{NS}}^{\max }$. Leveraging this correlation and the observed NS maximum mass distribution, $P(M_{\mathrm{NS}}^{\max }|\mathrm{EM})$, we derive the probability distribution function (PDF) for the maximum DM mass, $P(M_{\chi }^{\max }|\mathrm{EM})$, in DANSs. We find that at the 68% confidence level, $M_{\chi }^{\max }=0.15_{-0.05}^{+0.07}{M}_{\odot }$, which is confirmed to hold for a wide selection of dark matter EOS models. This quantitative constraint on the DM mass provides a critical prior for interpreting potential observational signatures of DANSs, such as anomalous tidal deformabilities and distinctive gravitational-wave signals.

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• Nuclear matter in neutron stars
• Particle dark matter
• Neutron stars & pulsars
• Astronomical masses & mass distributions
• Composition of astronomical objects