The market portfolio
In Finance class today, the market portfolio was discussed. The teacher skipped pretty lightly over the reason why the market portfolio could be used in place of the tangency portfolio in CAPM.
This reminded me of the paper The Significance of the Market Portfolio, Athanasoulis and Shiller (2000). From the paper: "In a simple general equilibrium exchange model of all possible markets to create, a market for shares in the world portfolio would be, by a social welfare criterion, a least important market to create, not a most important market (Theorem 2)."