The dark pool, which functions as an invisible order book separate from its public offering, is intended to provide a means for traders to move more than 50 BTC (roughly $12,500 at press time) on the market in relative secret.
Dark pools are used throughout the global financial system, though the practice has faced rising scrutiny from regulators, and banks have been penalized in the past for unlawful activity within the dark pools they operate. Supporters of dark pools say they help stabilize markets and reduce costs.
Full story: http://www.coindesk.com/bitcoin-exhange-kraken-dark-pool/