We propose a new way to search for (hidden) cool molecular hydrogen H2 in the Galaxy through diffractive and refractive effects: in favorable cases the light of a background star can be subject to stochastic fluctuations of the order of a few percent at a characteristic time scale of a few minutes. We tested the concept by observing stars in SMC and through nebulae with the NTT telescope. One star showed fluctuations compatible with a scintillation effect through a turbulent structure in B68 nebula. We could establish upper limits on the contribution of gas clumpuscules to the Galactic halo. We show that monitoring a few 106 star × hour with a > 4 m telescope and a fast readout camera should allow one to seriously constrain the contribution of turbulent molecular gas to the Galactic mass.