  The current cosmological model ($\Lambda$CDM) with the underlying FLRW metric
relies on the assumption of local isotropy, hence homogeneity of the Universe.
Difficulties arise when one attempts to justify this model as an average
description of the Universe from first principles of general relativity, since
in general, the Einstein tensor built from the averaged metric is not equal to
the averaged stress--energy tensor. In this context, the discrepancy between
these quantities is called "cosmological backreaction" and has been the subject
of scientific debate among cosmologists and relativists for more than $20$
years. Here we present one of the methods to tackle this problem, i.e.
averaging the scalar parts of the Einstein equations, together with its
application, the cosmological mass function of galaxy clusters.
