The Komar were a Delta Quadrant people of non-corporeal, trianic energy beings that inhabited a dark matter nebula. They were parasites who lived by absorbing the neural energy of other beings. They were able to inhabit the bodies of physical beings and had technology that can render a corporeal being non-corporeal, leaving their body essentially brain dead. In 2371, Chakotay was separated from his body and Tuvok was possessed when they encountered the Komar. (ST reference: Star Trek Cookbook,VOY episode: "Cathexis")