According to the modern theory of galaxy formation, galaxies form because of cooling of baryons and star formation at the centers of gigantic halos of dark matter.The cold neutral hydrogen (HI) layer of the galactic disk serves as an effective tracer of the underlying gravitational potential of the dark matter halo in nearby, edge-on spiral galaxies. In the first part of the talk, I will discuss how the density profile of the dark matter halo can be constrained by using the observed HI rotation curve and the HI layer thickness, as applied to the superthin low surface brightness galaxy UGC 7321, the Andromeda (M31) and our the Galaxy. In the second part, I will show how the superthin nature of the disk of the stars in the low surface brightness galaxy UGC7321 can be traced back to the presence of a dense and compact dark matter halo in this galaxy.