  Galaxy clusters trace the largest structures of the Universe and provide
ideal laboratories for studying galaxy evolution and cosmology. Clusters with
extended X-ray emission have been discovered at redshifts up to z ~ 2.5.
Meanwhile, there has been growing interest in hunting for protoclusters, the
progenitors of clusters, at higher redshifts. It is, however, very challenging
to find the largest protoclusters at early times when they start to assemble.
Here we report a giant protocluster of galaxies at redshift z = 5.7, when the
Universe was only one billion years old. This protocluster occupies a volume of
about 35x35x35 cubic co-moving megaparsecs. It is embedded in an even larger
overdense region with at least 41 spectroscopically confirmed, luminous
Lyman-alpha emitting galaxies (Lyman-alpha Emitters, or LAEs), including
several previously reported LAEs. Its LAE density is 6.6 times the average
density at z ~ 5.7. It is the only one of its kind in an LAE survey in four
square degrees on the sky. Such a large structure is also rarely seen in
current cosmological simulations. This protocluster will collapse into a galaxy
cluster with a mass of (3.6+/-0.9) x 10^{15} solar masses, comparable to those
of the most massive clusters or protoclusters known to date.
