  We investigate the influence of an interaction between dark energy and dark
matter upon the dynamics of galaxy clusters. We obtain the general
Layser-Irvine equation in the presence of interactions, and find how, in that
case, the virial theorem stands corrected. Using optical, X-ray and weak
lensing data from 33 relaxed galaxy clusters, we put constraints on the
strength of the coupling in the dark sector. We find that this coupling is
small but positive, indicating that dark energy can be decaying into dark
matter.
