The current of dark money that has flowed into the U.S. political system over the past three years -- as anonymously-funded outside groups have proliferated -- depends on secrecy. And yet, even though anonymous donors have become a commonplace part of national elections, it came as a shock last week when a little-known group with strong ties to the Koch brothers stepped forward and acknowledged itself as the source of nearly a quarter of a billion dollars given to conservative groups in 2012.