Dark pools are a type of private market that allow investors to trade without revealing their identities. Investment banks promoted them as a way to buy or sell large amounts of shares without moving the market, as transactions are only published once they’re completed.

On paper, dark pools were meant to protect investors from predatory high-frequency trading tactics. But in 2014, New York attorney general Eric Schneiderman accused Barclays of misleading investors, and allowing them to be lose out to professional high-frequency traders lurking in its dark pool.