Ricardo Hausmann defends dark matter in Martin Wolf's FT Economist's Forum (the section of the Hausmann and Sturzenegger paper on page 5 called "What is Behind Dark Matter" is a good introduction if you are unfamiliar with the dark matter concept):
FT Forum, by Ricardo Hausmann: Dear all: Martin Wolf is quick to dismiss the discussion of dark matter and side with Willem Buiter. However, Federico Sturzenegger and I still believe they miss the point big time. ... At this point of the debate I hope we all agree that there is a puzzle between the story that the current account deficit describes and what the financial income flows reflect. A correct descriptive explanation of this puzzle is that the rates of return on US liabilities is significantly smaller that the return on its assets. ...
OK, so the question is why US assets abroad earn more than US liabilities at home. ... The profession must offer an interpretation of this puzzle. Our interpretation is that the US has a comparative advantage in providing insurance, liquidity and innovation to the rest of the World, and that the rest of the world is happy to pay for this. Willem Buiter and Martin Wolf buy the seignorage argument but point out that the magnitude is too small to compensate for the US deficit. They jump from this conclusion