The links I pull up assume that the reader knows what null dust is. Which I don't. An explanation would be appreciated. The question came up when looking for an (equally absent) clear explanation of Vaidya spacetime. This, in turn, came up while looking at the article http://prola.aps.org/abstract/PRD/v70/i10/e101502; perhaps also could someone tell me whether this is just a rerun of the outmoded "white hole" construction?