  We present a detailed study of a rich galaxy cluster at z=2.38. We
demonstrate that this cluster contains large overdensities of damped Ly-alpha
absorption lines, of Ly-alpha emitting galaxies and of extremely red objects.
The overdensity of extremely red objects in this field demonstrates that many
are high z galaxies.
  The huge overdensities we measure for these three classes of object are much
larger than the mass overdensities of typical clusters at this redshift, as
predicted by CDM and related models. We suggest therefore that the distribution
of damped Ly-alpha absorption line systems, of Ly-alpha emitting galaxies and
of extremely red objects are all very strongly biassed, and that somehow a
small overdensity of mass has increased the fraction of baryons in collapsed
objects, in the volume occupied by the cluster, to close to unity (a factor of
~10 increase).
  We speculate that some unknown physical process, acting on the volume
occupied by our cluster, caused the normally diffuse ionised inter-galactic
medium to coalesce into small (< 10^8 Solar masses) blobs of neutral hydrogen,
which produce the Ly-alpha absorption-lines. Star formation occurred within
these blobs at z>5, enriching them with metals and producing stars, which after
several mergers and ~ 0.5 Gyr of passive evolution form the extremely red
objects. The Ly-alpha emitting galaxies are probably AGN, triggered perhaps by
mergers of the small blobs.
