After SN1a and WMAP observations during last decade, the discovery of the
accelerated expansion of the universe is a major challenge of
astro-particle physics and cosmology. There are currently three candidates
for the " Dark-Energy" which drives this accelerated expansion:
1) a non-zero cosmological constant, 2) an ultralight scalar field
(quintessence field), 3) modifications of Einstein's theory of gravity.
In this talk, we review briefly the main idea of three possible candidates
and their relatied phenomena. We discuss a way how we can determine the origin
of dark-energy in future experiments. As a special case, we investigate
interacting mechanism between the dark-energy and the dark-matter (neutrinos).
We study CMB-radiation and large scale structure of the Universe, and find
that evolution of neutrino masses is determined by quintessence, which is
responsible for the cosmic acceleration today. Finally, we discuss
prospects of dark-energy search for the forthcoming 20 years.