  Since there is no known symmetry in Nature that prevents a non-minimal
coupling between the dark energy (DE) and cold dark matter (CDM) components,
such a possibility constitutes an alternative to standard cosmology, with its
theoretical and observational consequences being of great interest. In this
paper we propose a new null test on the standard evolution of the dark sector
based on the time dependence of the ratio between the CDM and DE energy
densities which, in the standard $\Lambda$CDM scenario, scales necessarily as
$a^{-3}$. We use the latest measurements of type Ia supernovae, cosmic
chronometers and angular baryonic acoustic oscillations to reconstruct the
expansion history using model-independent Machine Learning techniques, namely,
the Linear Model formalism and Gaussian Processes. We find that while the
standard evolution is consistent with the data at $3\sigma$ level, some
deviations from the $\Lambda$CDM model are found at low redshifts, which may be
associated with the current tension between local and global determinations of
$H_0$.
