Resumo
Positive energy ray representations of the Poincar ́ group are naturally subdivided into threeeclasses according to their mass and spin content: m>0, m=0 finite helicity and m=0 infinite helicity.For a long time the localization properties of the massless infinite spin class remained unknown beforeit became clear that such matter does not permit compact spactime localization and its generatingcovariant fields are localized on semi-infinite spacelike strings.It is shown that such matter cannot interact with normal matter within the conceptual setting ofQFT since any such coupling leads to a total delocalization in higher perturbative orders. However,as any positive energy matter, it interacts with gravity. Its inert behavior with respect to ordinarymatter makes it an ideal candidate for dark matter.