  It is widely believed that structure in the Universe evolves hierarchically,
as primordial density fluctuations, amplified by gravity, collapse and merge to
form progressively larger systems. The structure and evolution of X-ray
clusters, however, seems at odds with this hierarchical scenario for structure
formation. Poor clusters and groups, as well as most distant clusters detected
to date, are substantially fainter than expected from the tight relations
between luminosity, temperature and redshift predicted by these models. Here we
show that these discrepancies arise because, near the centre, the entropy of
the hot, diffuse intracluster medium (ICM) is higher tha$ achievable through
gravitational collapse, indicating substantial non-gravitational heating of the
ICM. We estimate this excess entropy for the first time, and argue that it
represents a relic of the energetic winds through which forming galaxies
polluted the ICM with metals. Energetically, this is onl$ possible if the ICM
is heated at modest redshift ($z \ltsim 2$) but prior to cluster collapse,
indicating that the formation of galaxies precedes that of clusters and that
most clusters have been assembled very recently.
