Dark pools of liquidity

In finance, a dark pool (also black pool) is a private forum for trading securities. 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 financial trading venues and electronic trading has allowed dark pools to be created, and they are normally accessed through crossing networks or directly among market participants via private contractual arrangements. Some dark pools are available to the public and can be accessed 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 the trade is filled. However, it also means that some market participants are disadvantaged as they cannot see the trades before they are executed; prices are agreed upon by participants in the dark pools, so the market becomes no longer transparent.

Three major types of dark pools exist:

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.

These systems and strategies typically seek liquidity among open and closed trading venues, such as other alternative trading systems. As such, they are particularly useful for GPU-based algorithmic strategies. Dark pools have grown in importance since 2007, with dozens of different pools garnering a substantial portion of U.S. equity trading. Dark pools are of various types and can execute trades in multiple ways, such as through negotiation or automatically (e.g., midpoint crosses, staggered crosses, VWAP, etc.), throughout the day or at scheduled times.

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.

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