Is there a need for dark matter in galaxies?
  We present arguments indicating that galaxies and their clusters should be
considered as open developing systems. Galaxies interact with the intergalactic
medium and are not in a virial equilibrium (which is determined by the gravity
and rotation). In this case, the conventional interpretation of the rotation
curves of galaxy outer regions outside the visible stellar disk (i.e. the
presence of high mass dark matter haloes) may be erroneous. If there is an
accretion of the intergalactic medium in these regions, then the orbital
velocities of neutral hydrogen clouds are determined not only by the
gravitation of the mass inside their orbits. Galaxy clusters accrete the
material (intergalactic gas and galaxies) from filaments of the large-scale
structure, at the intersections of which they are located. Only their central
regions can approach the virial equilibrium. Therefore, the high velocities of
galaxies and the high temperatures of the intergalactic gas in a cluster do not
necessarily indicate the presence of a large mass of dark matter in the
cluster.
