  We investigate the consequences of an imperfect dark energy component on the
large scale structure. A phenomenological three parameter fluid description is
used to study the effect of dark energy on the cosmic microwave background
radiation (CMBR) and matter power spectrum. In addition to the equation of
state and the sound speed, we allow a nonzero viscosity parameter for the
fluid. Then anisotropic stress perturbations are generated in dark energy. In
general, we find that this possibility is not excluded by the present day
cosmological observations. In the simplest case when all of the three
parameters are constant, we find that the observable effects of the anisotropic
stress can be closely mimicked by varying the sound speed of perfect dark
energy. However, now also negative values for the sound speed, as expected for
adiabatic fluid model, are tolerable and in fact could explain the observed low
quadrupole in the CMBR spectrum. We investigate also structure formation of
imperfect fluid dark energy characterized by an evolving equation of state. In
particular, we study unified models of dark energy with dark matter, such as
the Chaplygin gas or the Cardassian expansion, with a shear perturbation
included. This can stabilize the growth of inhomogeneities in these models,
thus somewhat improving their compatibility with large scale structure
observations.
