  We present a simple cosmological model in which a single, non-minimally
coupled scalar field with a quartic potential is responsible for both inflation
at early times and acceleration at late times. Little or no fine tuning is
needed to explain why the present density of dark energy is comparable to that
of pressureless matter. Dark energy is identified with the potential of the
scalar field, which is sourced by the trace of the energy-momentum tensor. This
becomes significant when matter has decoupled from radiation and become fully
non-relativistic, so that
$\phi\propto\rho_m^{1/3}\propto\rho_{m,0}^{1/3}(a_0/a)\sim(10^{-120})^{1/3}(10^{10})\sim10^{-30}$
and $V\sim\phi^4\sim10^{-120}$ in Planck units, as observed.
