  The pure Brans-Dicke (BD) gravity with or without the cosmological constant
$\Lambda $ has been taken as a model theory for the dark matter. Indeed, there
has been a consensus that unless one modifies either the standard theory of
gravity, namely, general relativity, or the standard model for particle
physics, or both, one can never achieve a satisfying understanding of the
phenomena associated with dark matter and dark energy. Along this line, our
dark matter model in this work can be thought of as an attempt to modify the
gravity side alone in the simplest fashion to achieve the goal. Among others,
it is demonstrated that our model theory can successfully predict the emergence
of dark matter halo-like configuration in terms of a self-gravitating spacetime
solution to the BD field equations and reproduce the flattened rotation curve
in this dark halo-like object in terms of the non-trivial energy density of the
BD scalar field, which was absent in the context of general relativity where
Newton's constant is strictly a ``constant'' having no dynamics. Our model
theory, however, is not entirely without flaw, such as the prediction of
relativistic jets in all types of galaxies which actually is not the case.
