  The next generation of cosmic microwave background (CMB) experiments, galaxy
surveys, and high-redshift observations can potentially determine the nature of
the dark matter observationally. With this in mind, we introduce a
phenomenological model for a generalized dark matter (GDM) component and
discuss its effect on large-scale structure and CMB anisotropies. Specifying
the gravitational influence of the otherwise non-interacting GDM requires not
merely a model for its equation of state but one for its full stress tensor.
  From consideration of symmetries, conservation laws, and gauge invariance, we
construct a simple but powerful 3 component parameterization of these stresses
that exposes the new phenomena produced by GDM. Limiting cases include: a
particle component (e.g. WIMPS, radiation or massive neutrinos), a cosmological
constant, and a scalar field component. Intermediate cases illustrate how the
clustering properties of the dark matter can be specified independently of its
equation of state. This freedom allows one to alter the amplitude and features
in the matter power spectrum relative to those of the CMB anisotropies while
leaving the background cosmology fixed. Conversely, observational constraints
on such properties can help determine the nature of the dark matter.
