  X-ray observations with Chandra and XMM are providing valuable new
measurements of the baryonic and dark matter content of groups and clusters.
Masses of cD clusters obtained from X-ray and gravitational lensing studies
generally show good agreement, therefore providing important validation of both
methods. Gas fractions have been obtained for several clusters that verify
previous results for a low matter density (Omega_m ~0.3). Chandra has also
provided measurements of the mass profiles deep down into several cluster cores
and has generally found no significant deviations from CDM predictions in
contrast to the flat core density profiles inferred from the rotation curves of
low-surface brightness galaxies and dwarf galaxies; i.e., there is no evidence
for self-interacting dark matter in cluster cores. Finally, initial studies of
the iron and silicon abundances in centrally E-dominated groups show that they
have pronounced gradients from 1-2 solar values within the central 30-50 kpc
that fall to values of 0.3-0.5 solar at larger radii. The Si/Fe ratios are
consistent with approximately 80% of the metals originating from Type Ia
supernovae. Several cD clusters also display central Fe enhancements suggestive
of Type Ia supernova enrichment, though some have central dips that may provide
a vital clue for solving the cooling flow mystery.
