By Jenny Strasburg and Scott Patterson

Dark pools – private, off-exchange computerized trading platforms – can be darker than their customers ever realize, as the case of Pipeline Trading Systems LLC shows.

But that's only where the shadows start.

Some 33% of U.S. stock trading takes place away from exchanges, according to research firm Tabb Group. That's "up dramatically from 15% in 2008," Tabb Group says.

Dark pools account for only a small part of that jump. The much larger fraction comes from another growing area of off-exchange trading called "internalization." With internalization, big institutional investors match buy and sell orders internally, using their own inventory of stocks.