Giovanni Montani, Luis A. Escamilla, Nakia Carlevaro, Francesco Cianfrani

We investigate late-Universe dynamics in which the dark matter component is described by axion particles. The proposed framework departs from the standard $Λ$CDM paradigm due to a small fraction of axions driving the system away from thermal equilibrium. We analyze the evolution of the axion energy density using both a kinetic and a classical field approach, yielding an identical macroscopic evolution equation for the dark matter density. The resulting scenario modifies $Λ$CDM dynamics in the late Universe (specifically at $z \lesssim 1$), while asymptotically recovering the standard baseline at earlier cosmic epochs. We compare the theoretical predictions of our formulation against a comprehensive suite of late-Universe datasets. Our statistical analysis reveals that when the SH0ES local calibration is included, the collisional axion model becomes significantly favored over $Λ$CDM, yielding a best-fit Hubble constant of $H_0 \simeq 73~{\rm km\,s^{-1}\,Mpc^{-1}}$. Ultimately, this cosmological scenario successfully accommodates local distance-ladder measurements while maintaining excellent agreement with Baryon Acoustic Oscillation data from the DESI Collaboration.