  The surface brightness distribution in the majority of stellar galactic discs
falls off exponentially. Often what lies beyond such a stellar disc is the
neutral hydrogen gas whose distribution also follows a nearly exponential
profile at least for a number of nearby disc galaxies. Both the stars and gas
are commonly known to host lopsided asymmetry especially in the outer parts of
a galaxy. The role of such asymmetry in the dynamical evolution of a galaxy has
not been explored so far.
  Following Lindblad's original idea of kinematic density waves, we show that
the outer part of an exponential disc is ideally suitable for hosting lopsided
asymmetry. Further, we compute the transport of angular momentum in the
combined stars and gas disc embedded in a dark matter halo. We show that in a
pure star and gas disc, there is a transition point where the free precession
frequency of a lopsided mode, $\Omega -\kappa $, changes from retrograde to
prograde and this in turn reverses the direction of angular momentum flow in
the disc leading to an unphysical behaviour. We show that this problem is
overcome in the presence of a dark matter halo, which sets the angular momentum
flow outwards as required for disc evolution, provided the lopsidedness is
leading in nature. This, plus the well-known angular momentum transport in the
inner parts due to spiral arms, can facilitate an inflow of gas from outside
perhaps through the cosmic filaments.
