Brokerages that execute internal trades and so called "dark pools" -- off-exchange venues favored by institutional investors -- are driving the explosion in U.S. stock trading away from exchanges, says Tabb Group. In March, a record 37% of U.S. equities trading was executed away from the NYSE-Euronext (NYX), Nasdaq OMX Group (NDAQ) and other traditional exchanges, says Tabb Group, a markets consulting firm. The trading desks of brokerages, which