The quantum field theory predicts that vacuum energy should manifest as a repulsive field.
At the same time, vacuum energy should be gravitationally attractive.
So, attraction and repulsion should to an extent cancel each other. Why we cannot explain the small value of cosmological constant with this cancellation?
The net source of gravity (i.e the effective "mass density" in weak field Poisson's equation) is $E+3P$ where $E$ is the energy density and $P$ the pressure (these have the same units). The Lorentz invariant vacuum energy-momentum tensor of dark energy has $$ T_{\mu\nu}={\rm diag}(E,P,P,P) =E\, g_{\mu\nu} = E\, {\rm diag}(1,-1,-1,-1). $$ This means that $P=-E$ so the negative $P$ outweighs the positive and attractive $E$ to give an efective "mass" density of $-2E$ -- i.e repulsion.
QFT predicts an infinite value of the quantum vacuum energy (or less if we don't count the energies above the Planck energy). The biggest false prediction ever. Obviously, this can't be the case (as one can observe easily). However, renormalization comes to the rescue, which tames the vacuum energy. The cut-off energy is the Planck energy.
So the vacuum energy is zero. But, as one can read in the Wikipedia article about the quantum vacuum:
Quantum theory of the vacuum further stipulates that the pressure of the zero-state vacuum energy is always negative and equal in magnitude to ρ. Thus, the total is ρ + 3p = ρ − 3ρ = −2ρ, a negative value. If indeed the vacuum ground state has non-zero energy, the calculation implies a repulsive gravitational field, giving rise to an acceleration of the expansion of the universe. However, the vacuum energy is mathematically infinite without renormalization, which is based on the assumption that we can only measure energy in a relative sense, which is not true if we can observe it indirectly via the cosmological constant.
Mark that it's written: If indeed the vacuum ground state has non-zero energy. If, which means one isn't sure, but it can serve as an explanation for dark energy.
See also this article for further information.
