  This paper describes how the non-gravitational contribution to Galactic
Velocity Rotation Curves can be explained in terms of a negative Cosmological
Constant ($\Lambda$). It will be shown that the Cosmological Constant leads to
a velocity contribution proportional to the radii, at large radii, and
depending on the mass of the galaxy. This explanation contrasts with the usual
interpretation that this effect is due to Dark Matter halos. The velocity
rotation curve for the galaxy NGC 3198 will be analysed in detail, while
several other galaxies will be studied superficially. The Cosmological Constant
derived experimentally from the NGC 3198 data was found to be:$|\Lambda|_{Exp}=
5.0\times 10^{-56} cm^{-2}$. This compares favourably with the theoretical
value obtained from the Large Number Hypothesis of:
$|\Lambda|_{Theory}=2.1\times 10^{-56}cm^{-2}$. The Extended LNH is then used
to define other cosmological parameters: gravitational modification constant,
energy density, and the Cosmological Constant in terms of a fundamental length.
A speculative theory for the evolution of the Universe is outlined where it is
shown how the Universe can be defined, in any particular era, by two
parameters: the fundamental length and the energy density of the vacuum for
that epoch. The theory is applied to the time evolution of the universe where a
possible explanation for the $\rho_{Planck}/\rho_{\Lambda}^{QH} \approx
10^{120}$ problem is proposed. The nature of the ''vacuum'' is reviewed along
with a speculative approach for calculating the Cosmological Constant via
formal M-theory.The experimentally derived results presented in this paper
support a decelerating Universe, in contrast with recent indicationsfrom Type
Ia Supernovae experiments, for an accelerating Universe.
