We present the scientific case for a high resolution spectro-imager in the mid-IR that would be dedicated to the mapping of H2 emission in its four lowest rotational transitions, at 28.2, 17.0, 12.3 and 9.7 micron with a spectral resolution ~104 sufficient to provide kinematical distances in galaxies. The proposed instrument on a 2 m-class telescope will be most sensitive to H2 line emission simultaneously extended and structured at small scale, in gas at temperatures higher than 80 K. Colder H2, which may contribute most of the baryonic dark matter in galaxies, will be traced by the emission of glitters of warm H2 heated, throughout the medium, by the dissipation of omnipresent turbulence. The main scientific objectives are to (i) directly measure the mass and temperature distribution of the warm H2, far from star forming regions, in particular in low metallicity environments where the traditional tracers of H2 (CO, dust emission) fail, (ii) trace the dissipation of turbulence in the perspective of building a global view of the star formation process in galaxies and (iii) trace baryonic dark matter in the form of cold H2 in a large sample of galaxies along the Hubble sequence.