Is Dark Matter Really Matter?

Abstract

Abstract In the standard model of cosmology, it is assumed that dark matter is pressureless with equation of state w = 0 and dark energy has w  = −1. We test these assumptions jointly using DESI DR2 distance measurements, including the recent Ly α Alcock–Paczynski (AP) information, Dark Energy Survey (DES) supernovae, and two complementary cosmic microwave background (CMB) treatments. When constant w DM and w DE are varied together, we find w DM = 0.00096 8 − 0.000496 + 0.000501 and w DE = − 0.938 0 − 0.0262 + 0.0259 (68%). With an alternative CMB treatment that marginalizes over the lensing spectrum, the corresponding constraints are w DM = 0.00087 0 − 0.000410 + 0.000408 and w DE = − 0.935 3 − 0.0254 + 0.0258 . Both standard ΛCDM values are disfavored at approximately 2 σ in the joint extension. Neither parameter departs significantly from its standard value when only that parameter is varied. This behavior arises because late-time distances favor w DE  > −1, while maintaining the early-Universe physical matter density requires a compensating positive w DM , which changes the mapping to the matter density today. Allowing dynamical dark energy further highlights the complexity of the situation: phantom crossing for dark energy makes w DM  = 0 consistent with the data, whereas a positive w DM preference persists when crossing is forbidden. Interestingly, the Padé- w parameterization that provides a flexible description of a class of quintessence models (with no phantom crossing), along with w DM free, is even mildly favored over the phantom-crossing w 0 w a model according to both the best-fit χ 2 and the deviance information criterion (DIC) under both CMB treatments. One can conclude that the apparent preference for phantom crossing may instead reflect deviations in the dark-matter sector rather than dark-energy dynamics alone. With future high-redshift distance measurements, particularly improved Ly α observations, together with growth and lensing probes, these possibilities may be further clarified.