  We study the backreaction of free quantum fields on a flat Robertson-Walker
spacetime. Apart from renormalization freedom, the vacuum energy receives
contributions from both the trace anomaly and the thermal nature of the quantum
state. The former represents a dynamical realisation of dark energy, while the
latter mimics an effective dark matter component. The semiclassical dynamics
yield two classes of asymptotically stable solutions. The first reproduces the
concordance model in a suitable regime. The second lacks a classical
counterpart, but is in excellent agreement with recent observations.
