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.
Truly dark liquidity can be collected off-market in dark pools. 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.[7]
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 if they desire and are contractually obligated.[8]
Dark pools allow funds to line up and move large blocks of equities without tipping their hands as to what they are up to. Modern trading platforms and the lack of human interaction have reduced the time scale on market movements. This increased responsiveness of the price of an equity to market pressures has made it more difficult to move large blocks of stock without affecting the price.[9]
Dark pools are run by private brokerages which operate under fewer regulatory and public disclosure requirements than public exchanges.[10] Tabb Group estimates trading on the dark pools accounts for 32% of trades in 2012 vs 26% in 2008.
For an asset that can be only publicly traded, the standard price discovery process is generally assumed to ensure that at any given time the price is approximately "correct" or "fair". However, very few assets are in this category since most can be traded off market without printing the trade to a publicly accessible data source. As the proportion of the daily volume of the asset that is traded in such a hidden manner increases, the public price might still be considered fair. However, if public trading continues to decrease as hidden trading increases, it can be seen that the public price does not take into account all information about the asset (in particular, it does not take into account what was traded but hidden) and thus the public price may no longer be "fair".
Yet when trades executed in dark pools are incorporated into a post-trade transparency regime, investors have access to them as a part of a consolidated tape. This can aid price discovery because institutional investors who are reluctant to tip their hands in lit market still have to trade and thus a dark pool with post-trade transparency improves price discovery by increasing the amount of trading taking place.
http://en.wikipedia.org/wiki/Dark_pool
