  We present a statistical comparison of three different estimates of cluster
mass, namely, the dynamical masses obtained from the velocity dispersion of
optical galaxies, the X-ray masses measured from the temperature of X-ray
emitting gas under the assumption of isothermal hydrostatic equilibrium, and
the gravitational lensing masses derived from the strong/weak distortions of
background galaxy images. Using a sample of 29 lensing clusters available in
literature, we have shown that the dynamical masses are in agreement with the
gravitational lensing masses, while the X-ray method has systematically
underestimated cluster masses by a factor 2-3 as compared with the others.
These results imply that galaxies indeed trace the gravitational potential of
their clusters, and there is no bias between the velocities of the dark matter
particles and the galaxies in clusters. The X-ray cluster mass discrepancy is
probably from the simplification in the models for the X-ray gas distribution
and dynamical evolution.
