Dark Pools: Large trades by financial institutions that are offered away from public exchanges so that trades are anonymous. Benefits of a dark pool include: anonymous trading with minimal market impact, price improvement and opacity, lower transaction costs, less information “leakage” and access to as much liquidity as possible through technology. Some traders that use an investment strategy based on liquidity feel that dark pools should be publicized in order to ensure trading is “fair” for all parties involved.

Source: Wikipedia, PriceWaterhouseCoopers, Investopedia