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 \mathcal { 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 .