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In this paper, we consider the Universe at the late stage of its evolutionand deep inside the cell of uniformity. At these scales, the Universe is filledwith inhomogeneously distributed discrete structures (galaxies, groups andclusters of galaxies). Supposing that the Universe contains also thecosmological constant and a perfect fluid with a negative constant equation ofstate (EoS) parameter $\omega$ (e.g., quintessence, phantom or frustratednetwork of topological defects), we investigate scalar perturbations of the FRWmetrics due to inhomogeneities. Our analysis shows that, to be compatible withthe theory of scalar perturbations, this perfect fluid, first, should beclustered and, second, should have the equation of state parameter$\omega=-1/3$. In particular, this value corresponds to the frustrated networkof cosmic strings. Therefore, the frustrated network of domain walls with$\omega =-2/3$ is ruled out. A perfect fluid with $\omega =-1/3$ neitheraccelerates nor decelerates the Universe. We also obtain the equation for thenonrelativistic gravitational potential created by a system of inhomogeneities.Due to the perfect fluid with $\omega = -1/3$, the physically reasonablesolutions take place for flat, open and closed Universes. This perfect fluid isconcentrated around the inhomogeneities and results in screening of thegravitational potential.

Rigorous theoretical constraint on constant negative EoS parameter $\omega$ and its effect for the late Universe