  We present current and future constraints on equations of state for dark
sector perturbations. The equations of state considered are those corresponding
to a generalized scalar field model and time-diffeomorphism invariant $L(g)$
theories that are equivalent to models of a relativistic elastic medium and
also Lorentz violating massive gravity. We develop a theoretical understanding
of the observable impact of these models. In order to constrain these models we
use CMB temperature data from Planck, BAO measurements, CMB lensing data from
Planck and the South Pole Telescope, and weak galaxy lensing data from
CFHTLenS. We find non-trivial exclusions on the range of parameters, although
the data remains compatible with $w=-1$. We gauge how future experiments will
help to constrain the parameters. This is done via a likelihood analysis for
CMB experiments such as CoRE and PRISM, and tomographic galaxy weak lensing
surveys, focussing in on the potential discriminatory power of Euclid on mildly
non-linear scales.
