  We show that if dark energy evolves in time, its dynamical component could be
dominated by a bath of dark radiation. Within current constraints this
radiation could have up to $\sim 10^4$ times more energy density than the
cosmic microwave background. We demonstrate particular models in which a
rolling scalar field generates different forms of dark radiation such as hidden
photons, milli-charged particles and even Standard Model neutrinos. We find the
leading effect on the late-time cosmological expansion history depends on a
single parameter beyond $\Lambda$CDM, namely the temperature of the dark
radiation today. Cosmological observations of this modified expansion rate
could provide a striking signature of this scenario. The dark radiation itself
could even be directly detectable in laboratory experiments, suggesting a
broader experimental program into the nature of dark energy.
