  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.
