Dark Pool

What Is a Dark Pool?

A dark pool is a private venue for trading securities, most often stocks, in which orders are not shown publicly before they are executed. On a public exchange, bids and offers appear in a visible order book. In a dark pool, orders stay hidden from other participants until a match occurs. The name refers to this lack of pre-trade transparency, not to any illegal activity. Dark pools are a type of alternative trading system (ATS) and are usually run by brokerage firms or specialized operators.

How Dark Pools Work

Participants send orders into the pool, where a matching system pairs buy and sell orders. Many dark pools execute trades at the midpoint of the best publicly quoted bid and ask prices, so the trade price falls between the bid-ask spread. Once a trade is completed, it is reported to the public tape, usually shortly afterward. The market therefore learns about the trade even though the order itself was hidden.

Why Investors Use Them

Large institutional investors, such as pension funds and mutual funds, often need to buy or sell big blocks of shares. If such an order were displayed openly, other traders could move the price against it before it was filled. This cost is known as market impact. Dark pools allow large orders to be filled with less of that effect. Some trades also receive price improvement, meaning they execute at a price better than the displayed quote.

Advantages and Concerns

Advantages include reduced market impact, possible price improvement, and confidentiality. Critics point out that hidden orders make it harder for the public to see true supply and demand, which can weaken price discovery. Dark pools also draw scrutiny over conflicts of interest, such as operators trading against their own clients, and over how well trades compare with prices on public markets. In the United States, the Securities and Exchange Commission oversees these venues and requires trade reporting. Individual investors typically interact with dark pools only indirectly, through the order routing decisions of their brokers.