Peculiar motion of visible tracers of the dark universe, such as galaxies and 21cm emitting gas, induces a characteristic anisotropy in the otherwise isotropic 3D clustering of these tracers. This effect of redshift space distortion provides a promising tool to measure  the structure growth rate of the universe and hence to probe dark matter, dark energy and gravity. I will explain the physics of redshift space distortion,
review its cosmological applications, and elaborate on challenge and progress in theoretical modeling.