Cosmological Implications of X-ray Observations of Groups and Clusters
Richard Mushotzky 
NASA/GSFC 

Recent x-ray observations of groups have revolutionized our understanding 
of the
nature and distribution of the baryonic and dark matter mass in the local
universe and the evolution of metals. These ASCA and Rosat data have shown 
that
many poor groups, the "average" place in the universe, 
are dominated by dark matter and have a different abundance
pattern in the intergalactic gas than that seen in rich clusters. The lower
ratio of the
mass of metals to starlight in groups compared to clusters indicates
that either the groups have had much of their metals removed or
that the process of metal formation in groups is significantly
different than that in rich clusters. If the former is true it
indicates that the intergalactic medium is metal enriched and if the latter 
then
there have been strong variations in the efficiency of star formation on 
very large mass scales. 
A census of baryons in the local universe indicates that most of
the visible baryons are in the hot intergalactic gas in groups.
Integration of the mass function derived from x-ray imaging
spectroscopy indicates that most of the total visible mass
density in the local universe is also in groups with  minimum 
contribution to the closure density of x-ray luminous groups and
clusters of 10%.