what are dark pools of liquidity ?

Dark pools are private exchanges. They are also known as crossing networks.

Why Dark?
-They do not display liquidity details to the public i.e. Order Size, Price, Counterparty
-May display liquidity only to selected participants
-May publish IOI's only to selected participants
-May publish best bids and offers if market volume is more than certain percentage of total traded volume

Who run them
-Broker/Dealers
-Consortia
-Independent firms.

Need for Dark Pools
-Better access for liquidity for block trades or illiquid Stock.
-Minimize risk of information leakage i.e. anonymity or do not disclose trading intentions
-Reduce impact Cost.
-Reduce trading Cost.

Others
-Earn revenue by publishing post-trade information.
-Execution price defined by reference price in main market
-Increase revenue by executing order on own dark pool (i.e.Broker's) than on market