A young galaxy cluster in the old Universe
  Galaxies evolve from a blue star-forming phase into a red quiescent one by
quenching their star formation activity. In high density environments, this
galaxy evolution proceeds earlier and more efficiently. Therefore, local galaxy
clusters are dominated by well-evolved red, elliptical galaxies. The fraction
of blue galaxies in clusters monotonically declines with decreasing redshift,
i.e., the Butcher-Oemler effect. In the local Universe, observed blue fractions
of massive clusters are as small as $\lesssim$ 0.2. Here we report a discovery
of a \lq \lq blue cluster\rq \rq, that is a local galaxy cluster with an
unprecedentedly high fraction of blue star-forming galaxies yet hosted by a
massive dark matter halo. The blue fraction is 0.57, which is 4.0 $\sigma$
higher than those of the other comparison clusters under the same selection and
identification criteria. The velocity dispersion of the member galaxies is 510
km s$^{-1}$, which corresponds to a dark matter halo mass of
2.0$^{+1.9}_{-1.0}\times 10^{14}$ M$_{\odot}$. The blue fraction of the cluster
is more than 4.7 $\sigma$ beyond the standard theoretical predictions including
semi-analytic models of galaxy formation. The probability to find such a high
blue fraction in an individual cluster is only 0.003\%, which challenges the
current standard frameworks of the galaxy formation and evolution in the
$\Lambda$CDM Universe. The spatial distribution of galaxies around the blue
cluster suggests that filamentary cold gas streams can exist in massive halos
even in the local Universe. However these cold streams have already disappeared
in the theoretically simulated local universes.
