  Massive clusters of galaxies have been found as early as 3.9 Billion years
(z=1.62) after the Big Bang containing stars that formed at even earlier
epochs. Cosmological simulations using the current cold dark matter paradigm
predict these systems should descend from "proto-clusters" - early
over-densities of massive galaxies that merge hierarchically to form a cluster.
These proto-cluster regions themselves are built-up hierarchically and so are
expected to contain extremely massive galaxies which can be observed as
luminous quasars and starbursts. However, observational evidence for this
scenario is sparse due to the fact that high-redshift proto-clusters are rare
and difficult to observe. Here we report a proto-cluster region 1 billion years
(z=5.3) after the Big Bang. This cluster of massive galaxies extends over >13
Mega-parsecs, contains a luminous quasar as well as a system rich in molecular
gas. These massive galaxies place a lower limit of >4x10^11 solar masses of
dark and luminous matter in this region consistent with that expected from
cosmological simulations for the earliest galaxy clusters.
