There’s a good short post on TheConnversation.com by Richard Payne of Cass Business School in London about “Dark Pools” and how they work in modern finance.
Roughly speaking a dark pool is a trading system that does not publish information on outstanding orders to buy or sell. Thus one can place an order to sell, say, but not advertise the fact that you wish to sell to the rest of the world. Then, periodically, buy and sell orders are matched with one another at a price that is usually derived from a “lit” (that is, not dark) market. After the trade is completed, the details (price and size) are published. Note, though, that there is no guarantee that if you submit an order to a dark pool, a trade will result (there will only be an execution if an order on the other side of the market arrives).
Read more at: https://theconversation.com/explainer-what-are-dark-pools-22444