Zero-point energy of vacuum fluctuation as a candidate for dark energy versus a new conjecture of antigravity based on the modified Einstein field equation in general relativity

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.