  I review the dark energy solutions by a very light pseudoscalar called
"quintessential axion". For the explicit breaking terms, we consider both the
global anomaly U(1)$_{\rm global}\times$SU(2)$_W^2$ and the potential $\Delta
V$. At the field theory level, we will present a strategy for picking up one
such pseudoscalar among plenty of pseudoscalars. At string level, numerous
antisymmetric tensor fields are pseudoscalars. Including these, we present a
confining force example via a $Z_{12-I}$ orbifold compactification of SO(32)
heterotic string. In addition, we also aim to draw attention on almost massless
mesons of confining nonabelian gauge group, which can be another motivation for
introducing an additional confining force, to explain dark energy as the vacuum
energy of these mesons.
