  How and when did galaxies form and assemble their stars and stellar mass? The
answer to these questions, so crucial to astrophysics and cosmology, requires
the full reconstruction of the so called cosmic star formation rate density
(SFRD), i.e. the evolution of the average star formation rate per unit volume
of the universe. While the SFRD has been reliably traced back to 10-11 billion
years ago, its evolution is still poorly constrained at earlier cosmic epochs,
and its estimate is mainly based on galaxies luminous in the ultraviolet and
with low obscuration by dust. This limited knowledge is largely due to the lack
of an unbiased census of all types of star-forming galaxies in the early
universe. We present a new approach to find dust-obscured star-forming galaxies
based on their emission at radio wavelengths coupled with the lack of optical
counterparts. Here, we present a sample of 197 galaxies selected with this
method. These systems were missed by previous surveys at optical and
near-infrared wavelengths, and 22 of them are at very high redshift (i.e. z >
4.5). The contribution of these elusive systems to the SFRD is substantial and
can be as high as 40% of the previously known SFRD based on UV-luminous
galaxies. The mere existence of such heavily obscured galaxies in the first two
billion years after the Big Bang opens new avenues to investigate the early
phases of galaxy formation and evolution, and to understand the links between
these systems and the massive galaxies which ceased their star formation at
later cosmic times.
