Trading in the $12.7 trillion U.S. Treasuries market, once the domain of Wall Street's biggest banks, is increasingly dominated by firms most people have never heard of.

Determining which ones is largely guesswork, however, which is peculiar in this market—the deepest, most liquid market in the world, which sets the benchmark rates for everything from mortgages to corporate debt. The unwelcome secrecy is also a little strange considering how much more transparent the world's largest banks have become.

This week, Risk.net published a list it obtained ranking the top interdealer traders of Treasuries. It showed three Chicago-based firms, Jump Trading, Citadel Securities, and Teza Technologies, accounting for 51 percent of the volume executed by the top 10 firms on BrokerTec, a popular electronic debt-trading platform.