  A overview is given of the recent developments in the $U(1)_X$ Stueckelberg
extensions of the Standard Model and of MSSM where all the Standard Model
particles are neutral under the $U(1)_X$ but an axion which is absorbed is
charged under both $U(1)_X$ and $U(1)_Y$ and acts as the connector field
coupling the Standard Model sector with the Stueckelberg sector. Coupled with
the usual Higgs mechanism that breaks the $SU(2)_L\times U(1)_Y$ gauge
symmetry, this scenario produces mixings in the neutral gauge boson sector
generating an extra $Z'$ boson. The couplings of the extra $Z'$ to the Standard
Model particles are milli weak but its couplings to the hidden sector matter,
defined as matter that couples only to the gauge field of $U(1)_X$, can be of
normal electro-weak strength. It is shown that such extensions, aside from the
possibility of leading to a sharp $Z' $ resonance, lead to two new types of
dark matter: milli weak (or extra weak) and milli charged. An analysis of the
relic density shows that the WMAP-3 constraints can be satisfied for either of
these scenarios. The types of models discussed could arise as possible field
point limit of certain Type IIB orientifold string models.
