Overview

In finance, a dark pool (also black pool) is a private forum (alternative trading system or ATS) for trading securities, derivatives, and other financial instruments. Liquidity on these markets is called dark pool liquidity. The bulk of dark pool trades represent large trades by financial institutions that are offered away from public exchanges like the New York Stock Exchange and the NASDAQ, so that such trades remain confidential and outside the purview of the general investing public.

The fragmentation of electronic trading platforms has allowed dark pools to be created, and they are normally accessed through crossing networks or directly among market participants via private contractual arrangements. Generally, dark pools are not available to the public, but in some cases, they may be accessed indirectly by retail investors and traders via retail brokers.

One of the main advantages for institutional investors in using dark pools is for buying or selling large blocks of securities without showing their hand to others and thus avoiding market impact, as neither the size of the trade nor the identity are revealed until some time after the trade is filled. However, it also means that some market participants—retail investors—are disadvantaged, since they cannot see the orders before they are executed. Prices are agreed upon by participants in the dark pools, so the market is no longer transparent.

A 2025 study found that dark trading is harmful to financial markets, as it either reduced market efficiency or entailed welfare losses.

Dark pools are heavily used in high-frequency trading (HFT), which has also led to a conflict of interest for those operating dark pools due to payment for order flow and priority access. High frequency traders may obtain information from placing orders in one dark pool that can be used on other exchanges or dark pools. Depending on the precise way in which a "dark" pool operates and interacts with other venues, it may be considered, and indeed referred to by some vendors, as a "grey" pool.

History

The origin of dark pools dates back to 1979 when financial regulation changed in the United States that allowed securities listed on a given exchange to be actively traded off the exchange in which it was listed. Known as reg 19c3 the U.S. Securities and Exchange Commission passed the regulation which would start on April 26, 1979.

The new regulation allowed the emergence of dark pools through the 1980s that allowed investors to trade large block orders while retaining privacy and avoiding market impact. In 1986, Instinet started the first dark pool trading venue known as "After Hours Cross". However, it was not until the next year that ITG created the first intraday dark pool "POSIT". Both allowed large trades to be executed anonymously, which was attractive to sellers of large blocks of shares. For the next 20 years trades executed on dark pools represented a small fraction of the market, between 3–5% of all trades.

This was sometimes referred to as "upstairs trading".

The next big development in dark pools came in 2007 when the SEC passed Regulation NMS (National Market System), which allowed investors to bypass public exchanges to gain price improvements. The effect of this was to attract a number of new players to the market and a large number of dark pools were created over the next 10 years. This was spurred on with the improvements of technology and increasing speed of execution as high-frequency trading took advantage of these dark pools.

Operation

Truly dark liquidity can be collected off-market in dark pools using FIX and FAST protocol based APIs. Dark pools are generally very similar to standard markets with similar order types, pricing rules and prioritization rules. However, the liquidity is deliberately not advertised—there is no market depth feed. Such markets have no need of an iceberg-order type. In addition, they prefer not to print the trades to any public data feed, or if legally required to do so, will do so with as large a delay as legally possible—all to reduce the market impact of any trade.

Dark pools are often formed from brokers' order books and other off-market liquidity. When comparing pools, careful checks should be made as to how liquidity numbers were calculated—some venues count both sides of the trade, or even count liquidity that was posted but not filled.

Dark liquidity pools offer institutional investors many of the efficiencies associated with trading on the exchanges' public limit order books but without showing their actions to others. Dark liquidity pools avoid this risk because neither the price nor the identity of the trading company is displayed.

Dark pools are recorded to the national consolidated tape. However, they are recorded as over-the-counter transactions. Therefore, detailed information about the volumes and types of transactions is left to the crossing network to report to clients only if they desire or are contractually obliged to do so.

Iceberg orders

Some markets allow dark liquidity to be posted inside the existing limit order book alongside public liquidity, usually through the use of iceberg orders. Iceberg orders generally specify an additional "display quantity"—i.e., smaller than the overall order quantity. The order is queued along with other orders but only the display quantity is printed to the market depth. When the order reaches the front of its price queue, only the display quantity is filled before the order is automatically put at the back of the queue and must wait for its next chance to get a fill.

Such orders will, therefore, get filled less quickly than the fully public equivalent, and they often carry an explicit cost penalty in the form of a larger execution cost charged by the market. Iceberg orders are not truly dark either, as the trade is usually visible after the fact in the market's public trade feed.

Price discovery

If an asset can be traded only publicly, the standard price discovery process has the best chance of making the public price approximately "correct" or "fair". However, very few assets are in this category, since most can be traded off market without revealing the trade publicly. As long as non-public trades are only a small fraction of trading volumes, the public price might still be considered fair. However, the greater the proportion of trading volume that happens non-publicly, the less confident we can be that the public price is "fair".

To lessen this adverse impact on price discovery, off-market venues can still report consolidated data on trades publicly. By this route, the trades occurring in dark pools can continue to contribute to price discovery, albeit with a little delay.