After inflation, the Universe passed through radiation dominated (RD) and matter dominated (MD) era during both of which the evolution of the scale factor (or the Hubble parameter) was decreasing with time as $H\sim t^{-1}$. These were phases of cosmic deceleration. However, the present Universe is undergoing an accelerated expansion. Whatever agent causes this expansion is termed as dark energy. It may be constant, or perhaps, it has been evolving with time.
If we assume that dark energy was always in the form of a constant energy density which doesn't dilute with expansion- a cosmological constant, why didn't it play a dominant role in the RD and MD era?