Many new and enigmatic high redshift galaxies that are intensively forming stars have been discovered using ESA’s Planck and Herschel satellites . These galaxies occur in clumps – and could be the long-sought formation phase of galaxy clusters. Some appear very bright, and have been found to be gravitationally lensed galaxies. These rapidly star-forming galaxies could help solve a central problem in cosmology: how did the large scale structure of galaxies form?