= Solution
For a relaxed cluster, galaxy motions act as tracers of its <gravitational potential>. The <virial theorem> gives a dynamical mass of order
$$
\boxed{M_{\rm dyn}\sim\frac{R\sigma_v^2}{G},}
$$
with a profile-dependent coefficient and a projection correction because the observed <velocity dispersion> is usually only the line-of-sight component. For an isotropic uniform-sphere estimate using its one-dimensional dispersion, $T=3M\sigma_v^2/2$ and $W=-3GM^2/(5R)$ give $M=5R\sigma_v^2/G$. Measured dispersions require much more gravitating mass than the luminous <galaxies> provide, hence large cluster <mass-to-light ratios>. Interlopers, orbital anisotropy and departures from equilibrium are systematic uncertainties, so the independent gas and lensing measurements matter.
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