  A smooth unclustered dark matter component with negative presure could
reconcile a flat universe with the many observations that find a density in
ordinary, clustered matter well below the critical density and also explain the
recent high redshift supernova data suggesting that the expansion of the
universe is now accelerating. For a perfect fluid negative presure leads to
instabilities that are most severe on the shortest scales. However, if instead
the dark matter is a solid, with an elastic resistance to pure shear
deformations, an equation of state with negative presure can avoid these short
wavelength instabilities. Such a solid may arise as the result of different
kinds of microphysics. Two possible candidates for a solid dark matter
component are a frustrated network of non-Abelian cosmic strings or a
frustrated network of domain walls. If these networks settle down to an
equilibrium configuration that gets carried along and stretched by the Hubble
flow, equations of state result with $w=-1/3$ and $w=-2/3,$ respectively. One
expects the sound speeds for the solid dark matter component to comprise an
appreciable fraction of the speed of light. Therefore, the solid dark matter
does not cluster, expect on the very largest scales, accessible only through
observing the large-angle CMB anisotropy. In this paper we develop a
generally-covariant, continuum description for the dynamics of a solid dark
matter component. We derive the evolution equations for the cosmological
perturbations in a flat universe with CDM+(solid) and compute the resulting
large-angle CMB anisotropy. The formalism presented here applies to any
generalized dark matter with negative pressure and a non-dissipative resistance
to shear.
