Dark Pools

Trading platforms where buyers and sellers place anonymous orders (off of an exchange) to obtain price improvement (often executed at the midpoint between the bid and the ask price), better liquidity and not move the price if a large block of stocks is being traded. They offer institutional investors many of the efficiencies associated with trading on the exchanges' public limit order books but without showing neither the price nor the identity of the trading company.
Critics state that individual investors should have equal access to stock-market information that larger institutions, with access to dark pools, enjoy. In Europe dark pools make up some 4 % of equity trading, in the US 11%.