Faced by recent evidence for a flat universe dominated by dark energy , cosmologists grapple with deep cosmic enigmas such as the cosmological constant problem , extreme fine-tuning and the cosmic coincidence problem . The extent to which we observe the dimming of distant supernovae suggests that the cosmic acceleration is as least as severe as in cosmological constant models . Extrapolating this to our cosmic future implies terrifying visions of either a cold and empty universe or an explosive demise in a “ Big Rip. ” We construct a class of dynamical scalar field models of dark energy and dark matter . Within this class we can explain why supernovae imply a cosmic equation of state w \lesssim - 1 , address fine tuning issues , protect the universe from premature acceleration and predict a constant fraction of dark energy to dark matter in the future ( thus solving the coincidence problem ) , satisfy the dominant energy condition , and ensure that gravitationally bound objects remain so forever ( avoid a Big Rip ) . This is achieved with a string theory inspired Lagrangian containing standard kinetic terms , exponential potentials and couplings , and parameters of order unity .