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 ‘ ‘ blue cluster ’ ’ , 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 .