When Cheryl Cargie, head trader at Ariel Investments in Chicago, decided last month to buy 1.3 million shares of a mid-cap stock listed on the New York Stock Exchange, she spread orders among several "dark pools," the secretive electronic trading networks that match buyers and sellers anonymously.

The pools are booming in popularity as big institutional investors look for ways to trade blocks of stock without triggering ripples in the share price, as can happen on traditional stock markets such as the NYSE and Nasdaq Stock Market.