It is fascinating that most many-body systems, if unperturbed, tend to relax towards thermal equilibrium dynamics. I will discuss a recent result showing that quantum equilibrium dynamics can be elevated to the rank of a universal (model-independent) symmetry of Keldysh field theories. This fundamental symmetry imposes strong constraints on the equilibrium correlation functions. But more importantly, this allows to study non-equilibrium dynamics as symmetry-breaking processes, providing important clues on the so-far poorly understood production of entropy in quantum mechanical systems.