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TheCDM paradigm of structure formation is successful at recovering the basic skeletal structure of the universe -- the large-scale distribution of galaxies. However, the agreement between theory and observation is less secure when this model is applied to galactic (and sub-galactic) scales. The "missing satellites problem" refers to the excess of predicted CDM sub-structure relative to observed Local Group dwarf galaxies. Recent discoveries of dark-matter dominated dwarf galaxies, some fainter than some star clusters, makes one wonder whether there may be a population of faint dwarf galaxies, lurking just beyond our reach. The extended atomic hydrogen disks of galaxies provide an unique probe of galaxy evolution. They are ideal tracers of tidal interactions with satellites and the galactic gravitational potential well. We have recently developed a method whereby one can infer the mass, and relative position (in radius and azimuth) of satellites from analysis of observed disturbances in outer gas disks, without requiring knowledge of their optical light. I will present the proof of principle of this method by applying it to galaxies with known optical companions. I will also present our earlier prediction for a dim and yet undiscovered companion of the Milky Way. I will end by presenting preliminary work on the application of this method to characterize the density profile of the dark matter halo in spiral galaxies.