Context : Sulphur is one of the most abundant elements in the Universe .
Surprisingly , sulphuretted molecules are not as abundant as expected in the interstellar medium and the identity of the main sulphur reservoir is still an open question .

Aims : Our goal is to investigate the H _ { 2 } S chemistry in dark clouds , as this stable molecule is a potential sulphur reservoir .

Methods : Using millimeter observations of CS , SO , H _ { 2 } S , and their isotopologues , we determine the physical conditions and H _ { 2 } S abundances along the cores TMC 1-C , TMC 1-CP , and Barnard 1b .
The gas-grain model Nautilus is used to model the sulphur chemistry and explore the impact of photo-desorption and chemical desorption on the H _ { 2 } S abundance .

Results : Our modeling shows that chemical desorption is the main source of gas-phase H _ { 2 } S in dark cores .
The measured H _ { 2 } S abundance can only be fitted if we assume that the chemical desorption rate decreases by more than a factor of 10 when n _ { H } > 2 \times 10 ^ { 4 } .
This change in the desorption rate is consistent with the formation of thick H _ { 2 } O and CO ice mantles on grain surfaces .
The observed SO and H _ { 2 } S abundances are in good agreement with our predictions adopting an undepleted value of the sulphur abundance .
However , the CS abundance is overestimated by a factor of 5 - 10 .
Along the three cores , atomic S is predicted to be the main sulphur reservoir .

Conclusions : The gaseous H _ { 2 } S abundance is well reproduced , assuming undepleted sulphur abundance and chemical desorption as the main source of H _ { 2 } S. The behavior of the observed H _ { 2 } S abundance suggests a changing desorption efficiency , which would probe the snowline in these cold cores .
Our model , however , highly overestimates the observed gas-phase CS abundance .
Given the uncertainty in the sulphur chemistry , we can only conclude that our data are consistent with a cosmic elemental S abundance with an uncertainty of a factor of 10 . \left . \right .