Positive energy ray representations of the Poincaré group are naturally subdivided into three classes according to their mass and spin content: m>0,m=0 finite helicity and m=0 infinite spin. For a long time the localization properties of the massless infinite spin class remained unknown, until it became clear that such matter does not permit compact spacetime localization and its generating covariant fields are localized on semi- infinite space-like strings. Using a new perturbation theory for higher spin fields we present arguments which support the idea that infinite spin matter cannot interact with normal matter and we formulate conditions under which this also could happen for finite spin s>1 fields. This raises the question of a possible connection between inert matter and dark matter. Dedicated to the memory of Robert Schrader.
