  In order to clarify why the zero-point energy associated with the vacuum
fluctuations cannot be a candidate for the dark energy in the universe, a
comparison with the Casimir effect is analyzed in some detail. A principle of
epistemology is stressed that it is meaningless to talk about an absolute
(isolated) thing. A relative thing can only be observed when it is changing
with respect to other things. Then a new conjecture of antigravity --the
repulsive force between matter and antimatter derived from the modified
Einstein field equation in general relativity-- is proposed. this is due to the
particle-antiparticle symmetry based on a new understanding about the essence
of special relativity. Its possible consequences in the theory of cosmology are
discussed briefly, including a new explanation for the accelerating universe
and gamma-ray-bursts.
