  In most cosmological models, the equation of state of the dark matter is
assumed to be zero, which means that the dark matter is pressure-less or cold.
While this hypothesis is based on the abundance of cold dark matter in the
universe, however, there is no compelling reason to assume that the equation of
state of dark matter is exactly zero. A more general approach would be to allow
for a range of values for the dark matter equation of state and use the
observational data to determine which values are most likely. With the
increasing accuracy of experimental data, we have chosen to explore the
possibility of interacting non-cold dark matter $-$ vacuum scenario, where the
equation of state of the dark matter is constant but can take different values
within a specific range. Using the Cosmic Microwave Background (CMB)
anisotropies and the CMB lensing reconstruction from the Planck legacy release,
plus other non-CMB measurements, namely, the baryon acoustic oscillations
distance measurements, and the Pantheon catalogue from Type Ia Supernovae, we
have analyzed this scenario and found that a non-zero value for the dark matter
equation of state is preferred with a confidence level of over 68\%. While this
is not significant by itself, however, it does suggest that investigating the
possibility of non-cold dark matter in the universe is worth exploring further
to gain a better understanding of the nature of dark matter.
