  Over the past decades, rest-frame ultraviolet (UV) observations have provided
large samples of UV luminous galaxies at redshift (z) greater than 6, during
the so-called epoch of reionization. While a few of these UV identified
galaxies revealed significant dust reservoirs, very heavily dust-obscured
sources at these early times have remained elusive. They are limited to a rare
population of extreme starburst galaxies, and companions of rare quasars. These
studies conclude that the contribution of dust-obscured galaxies to the cosmic
star formation rate density at $z>6$ is sub-dominant. Recent ALMA and Spitzer
observations have identified a more abundant, less extreme population of
obscured galaxies at $z=3-6$. However, this population has not been confirmed
in the reionization epoch so far. Here, we report the discovery of two
dust-obscured star forming galaxies at $z=6.6813\pm0.0005$ and
$z=7.3521\pm0.0005$. These objects are not detected in existing rest-frame UV
data, and were only discovered through their far-infrared [CII] lines and dust
continuum emission as companions to typical UV-luminous galaxies at the same
redshift. The two galaxies exhibit lower infrared luminosities and
star-formation rates than extreme starbursts, in line with typical star-forming
galaxies at $z\sim7$. This population of heavily dust-obscured galaxies appears
to contribute 10-25 per cent to the $z>6$ cosmic star formation rate density.

Astrophysics of Galaxies