We show how $\varsigma $ , the radial location of the minimum of the
differential radial mass profile ${M}^{\prime}(r)$ of a galaxy cluster, can probe
the theory of gravity. We derive ${M}^{\prime}(r)$ of the dark matter halos of
galaxy clusters from N-body cosmological simulations that implement two
different theories of gravity: standard gravity in the $\mathrm{\Lambda}$ CDM model and
$f(R)$ . We extract 49169 dark matter halos in 11 redshift bins in the range
$0\le z\le 1$ and in three different mass bins in the range
$0.9