  We propose a heuristic unification of dark matter and dark energy in terms of
a single dark fluid with a logotropic equation of state $P=A\ln(\rho/\rho_P)$,
where $\rho$ is the rest-mass density, $\rho_P$ is the Planck density, and $A$
is the logotropic temperature. The energy density $\epsilon$ is the sum of a
rest-mass energy term $\rho c^2$ mimicking dark matter and an internal energy
term $u(\rho)=-P(\rho)-A$ mimicking dark energy. The logotropic temperature is
approximately given by $A \simeq
\rho_{\Lambda}c^2/\ln(\rho_P/\rho_{\Lambda})\simeq\rho_{\Lambda}c^2/[123
\ln(10)]$, where $\rho_{\Lambda}$ is the cosmological density. More precisely,
we obtain $A=2.13\times 10^{-9} \, {\rm g}\, {\rm m}^{-1}\, {\rm s}^{-2}$ that
we interpret as a fundamental constant. At the cosmological scale, this model
fullfills the same observational constraints as the $\Lambda$CDM model.
However, it has a nonzero velocity of sound and a nonzero Jeans length which,
at the beginning of the matter era, is about $\lambda_J=40.4\, {\rm pc}$, in
agreement with the minimum size of the dark matter halos observed in the
universe. At the galactic scale, the logotropic pressure balances gravitational
attraction and solves the cusp problem and the missing satellite problem. The
logotropic equation of state generates a universal rotation curve that agrees
with the empirical Burkert profile of dark matter halos up to the halo radius.
In addition, it implies that all the dark matter halos have the same surface
density $\Sigma_0=\rho_0 r_h=141\, M_{\odot}/{\rm pc}^2$ and that the mass of
dwarf galaxies enclosed within a sphere of fixed radius $r_{u}=300\, {\rm pc}$
has the same value $M_{300}=1.93\times 10^{7}\, M_{\odot}$, in remarkable
agreement with the observations.
