Newtonian gravity and its relativistic extension, the general
theory of relativity, are extremely well tested in our solar system.
Both proposals were built with solar system dynamics provided in
Kepler's laws for the motion of planets. When the scales of mass
and length associated to a particular astrophysical system greatly
exceed the solar system values in such a way that the gravitational
acceleration on a given test particle is less or about 10^{-10} m /
s^2, either non-baryonic dark matter is required or Kepler's third
law of motion requires a modification (the Tully-Fisher law). I
will discuss non-relativistic and relativistic developments for
extending gravity based on the Tully-Fisher law, which in its simplest
form yields the basic MOdified Newtonian Dynamics (MOND) formula. I will
also show that this prescription has so far been adequate to explain
dynamics of systems from scales to solar system up to galaxy clusters
and the accelerated expansion of the universe. Also, I will explain how
these ideas are in very good agreement with the deflection of light and
gravitational lensing in individual, groups and clusters of galaxies.