Dark energy - a homogeneously distributed cosmic energy density - seems to
explain various cosmological observations. The anisotropies in the microwave
background radiation, the formation of structure and the late time
acceleration of the Hubble expansion fit into a consistent picture.
The role and origin of dark energy are among the greatest mysteries in
fundamental physics, touching the question of unification of gravity with the
fundamental quantum interactions.
We discuss quintessence - a dynamical form of dark energy - and
possible signatures distinctive from a cosmological constant. In particular,
an "early dark energy " may influence the cosmological evolution and the
growth of structures already at high redshift.
Quintessence could be related to a new "fundamental" macroscopic force and
induce a small time variation of fundamental constants.