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Electromagnetic cascading of TeV-band gamma-ray emission from distant blazars is a means to investigate the amplitude of magnetic field in the voids of intergalactic space. The flux of cascade emission from some objects is weaker than it should be, leaving two interpretation. The magnetic field may be strong enough to deflect the electron-positron pairs out of the line of sight. Alternatively plasma instabilities might drain the energy of the pairs. We present the current state of research and the most recent results, indicating that in many cases plasma instabilities provide sufficiently strong energy losses, but substantial uncertainty remains. Modelling of the late-time intensity spectrum of plasma waves indicates a highly nonlinear behaviour that can be significantly modified by seemingly subdominant processes.