  Galaxies are gigantic physical systems having a typical size of many tens of
thousands of light years. Thus any change at the center of the galaxy will
affect the rim only tens of millennia later. Those retardation effects seems to
be ignored in present day modelling used to calculate rotational velocities of
matter in the outskirts of the galaxy and the surrounding gas. The significant
discrepancies between the velocities predicted by Newtonian theory and observed
velocities are usually handled by either assuming an unobservable type of
matter denoted "dark matter" or by modifying the laws of gravity (MOND as an
example). Here we will show that considering general relativistic effects
without neglecting retardation one can explain the apparent excess matter
leading to gravitational lensing in both galaxies and galaxy clusters.
