  We propose to solve the dark energy and the coincidence problems by
postulating the massless sector of closed strings. This sector constitutes the
gravitational multiplet of string theory and, when applied to four-dimensional
cosmology, predicts that the expansion of an open Universe defined in string
frame is readily accelerating. We confront the prediction with the late-time
cosmological data of Type Ia supernovae and quasar absorption spectrum, which
probe the evolutions of the Hubble parameter and possibly the fine-structure
constant. We report that these observations are in admirable agreement with the
prediction without any dark sector and coincidence problem. We estimate the
Hubble constant, $H_{0}\simeq 71.2\pm 0.2\,\mathrm{km/s/Mpc}$.
