We propose a new way to search for (hidden)cool molecular hydrogen H2 in the Galaxy through diffractive and refractive effects: Stars twinkle because their light crosses the atmosphere. The same phenomenon is expected on a longer time scale when the lightof a remote star crosses an interstellar turbulent molecular cloud, but it has never been observed at optical wavelengths. Our simulations and test observations show that in favourable cases, the light of a background star can be subject to stochastic fluctuations on the order of a few percent at a characteristic time scale of a few minutes. I will discuss the unique potential of the LSST movie-mode to search and study such interstellar scintillation due to turbulent clouds through highly sampled light-curves. Since scintillation is expected to be a rare process (optical depth < 10 -3) and needs highly sampled light curves of small apparent size stars (i.e. faint stars), the--wide, deep and fast--LSST is the ideal device for this science. I will show that a modest amount of a few nights of observation time with LSST can significantly constrain the last unknown baryonic contribution to the Galactic mass.
