  We review the underpinnings of the standard Newton-Einstein theory of
gravity, and identify where it could possibly go wrong. In particular, we
discuss the logical independence from each other of the general covariance
principle, the equivalence principle and the Einstein equations, and discuss
how to constrain the matter energy-momentum tensor which serves as the source
of gravity. We identify the a priori assumption of the validity of standard
gravity on all distance scales as the root cause of the dark matter and dark
energy problems, and discuss how the freedom currently present in gravitational
theory can enable us to construct candidate alternatives to the standard theory
in which the dark matter and dark energy problems could then be resolved. We
identify three generic aspects of these alternate approaches: that it is a
universal acceleration scale which determines when a luminous Newtonian
expectation is to fail to fit data, that there is a global cosmological effect
on local galactic motions which can replace galactic dark matter, and that to
solve the cosmological constant problem it is not necessary to quench the
cosmological constant itself, but only the amount by which it gravitates.
