  Gravitational merging (or clustering) of cosmic objects is regarded as a
possible source of the extra-acceleration of the universe at large scale. The
merging/clustering of cosmic objects introduces a correction term in the
equation of state for the effective present cosmic fluid in the form of
$P=w\rho+b\rho^2$. As a result, an alternative relation for the energy density
includes over and under-dense regions is obtained that coincide with the
conventional relation in the standard limit. By analogy with bubbles, the
under-dense regions (voids) in the cosmic fluid is shown to provide the needed
negative pressure. Invoking the observational constraint for the dark energy
equation of state $w$, we show that the merging of voids will act as a possible
source of extra-accelerating at large scale in comparison with non-merging
cosmic gas.
