Examining a new form of non-standard dark matter using DESI DR2 data

In this work, we propose a non-standard dark matter (NSDM) model in which the equation of state (EoS) of dark matter (DM) is parameterized as wdm = w2a2, and this DM model is motivated by the idea that DM must become cold dark matter (CDM) in the neighborhood of the scale factor a = 0, which implies that both the EoS of DM, wdm, and its derivative with respect to the scale factor, dwdm/da, vanish at a = 0. By incorporating the latest cosmological datasets — including the Planck2018 Cosmic Microwave Background (CMB) distance priors, the Baryon Acoustic Oscillation measurements from the Data Release 2 of the Dark Energy SpectroscopicInstrument (DESI), together with three independent Type Ia Supernova datasets, namely the Dark Energy Survey Year 5 (DESY5) compilation, the Union3 compilation, and the PantheonPlus sample — we constrain the Λw2DM, ww2DM, and w0waw2DM models, which are constructed by replacing CDM with NSDM in the ΛCDM, wCDM, and w0waCDM models, respectively. We find that there is a preference for a negative DM EoS at more than the 3σ confidence level for the data combinations CMB+DESI+Union3 and CMB+DESI+DESY5. Moreover, for all data combinations, replacing CDM with NSDM in the wCDM and w0waCDM models significantly reduces the probability of violating the null energy condition. Furthermore, both ww2DM and w0waw2DM are favored over ΛCDM with a significance comparable to that of the w0waCDM model.