  The "dark age" of the Universe is generally pointed out as the period between
the recombination epoch and the horizon of current observations (z=5-6). The
arrow of time in the cosmic history describes the progression from simplicity
to complexity, because the present Universe is clumpy and complicated unlike
the homogeneous early Universe. Thus it is crucial to know the nature of the
constituents, in order to understand the conditions of the formation of the
first bound objects. In this paper we analyse the chemical history of this
"dark age" through the creation of the primordial nuclei to the formation of
the first atoms and molecules. Then we will describe the consequences of the
molecular formation on the birth of the proto-objects. In this context we will
mention the important works of Dennis W. Sciama who influenced a large number
of theorists -cosmologists and astronomers- on this new field of research
dedicated to primordial molecules.
