Investment Technology Group (ITG), a U.S. stock broker, paid a $20.3 million fine for running a secret operation named Project Omega to take advantage of “dark pools” trading orders made by its clients. Experts say that Barclays and Credit Suisse may also soon pay fines for dark pools trading scams.Most traders buy and sell stocks on public stock exchanges – but “dark pools” are essentially private stock markets located inside big banks that allow clients to place secret orders. Such systems have been in existence for 30 years and have become increasingly popular among institutional buyers who want to hide their identity or ensure that a large order does not cause the price to fluctuate. There are 45 such registered dark pools which now constitute as much as 40 percent of overall stock market trading in the U.S., according to the Tabb Group, a market research firm.