  In this letter, we estimate how much starlight was absorbed by dust and
released in the infrared during the formation of galaxies. New constraints are
set on this so-far hidden history by the recent detection of the Cosmic
Infrared Background (CIB) built up from the accumulated light of faint sources
(and revisited here). By means of a semi-analytical model of hierarchical
galaxy formation, we estimate the population of dust-enshrouded starburst
galaxies fitting the CIB. It is argued that a deep ISO survey at 175 micron
could already detect these sources and help close the gap in our understanding
of the history of light in the Universe.
