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Computer Trading and Dark Pools
Bob the Super Hamste writes "CNN Money has an article on computerized trading; specifically, the non-public markets that are often used to execute orders. The company that the article discusses executes 1/8 of all stock trades in the U.S., or about 900 million trades a day. For comparison, the NYSE executes about 700 million trades. The article discusses 'dark pools,' or private markets where quotes aren't disclosed to the broader public markets. If the company is unable to fill an order from within its own dark pool, it will submit the order to the broader public market (13 public exchanges), as well as up to 20 other private dark pools. The quotes offered by the private dark pools, by law, have to be the same or a better quote than those offered on public exchanges. There have been recent questions about whether the quotes provided by dark pools have been the best for customers and there is a current investigation by FINRA into the methods used by market makers and dark pool operators to fill orders."
I worked at a dark pool.
When a whale buys or sells a sizable amount of stock in the public market it moves the price. When they execute the trade it doesn't happen all at once but in blocks. When bids and offers are made other players in the market see it and they try to jump on. This moves the price. The whale would like the price not to move so they can maximize profit. When trades are executed in a dark pool the market doesn't see the trades until they clear at the end of the day. Who trades in a dark pool you might ask? Other whales. Stocks traded in a dark pool are usually fairly distributed between groups of buyers and sellers so no one trading party has an advantage.