  We discuss some outstanding open questions regarding the validity and
uniqueness of the standard second order Newton-Einstein classical gravitational
theory. On the observational side we discuss the degree to which the realm of
validity of Newton's Law of Gravity can actually be extended to distances much
larger than the solar system distance scales on which the law was originally
established. On the theoretical side we identify some commonly accepted but
actually still open to question assumptions which go into the formulating of
the standard second order Einstein theory in the first place. In particular, we
show that while the familiar second order Poisson gravitational equation (and
accordingly its second order covariant Einstein generalization) may be
sufficient to yield Newton's Law of Gravity they are not in fact necessary. The
standard theory thus still awaits the identification of some principle which
would then make it necessary too. We show that current observational
information does not exclusively mandate the standard theory, and that the
conformal invariant fourth order theory of gravity considered recently by
Mannheim and Kazanas is also able to meet the constraints of data, and in fact
to do so without the need for any so far unobserved non-luminous or dark
matter.
