Geez. Don't ask a doctor that question. The wiki: dark liquidity looks pretty good to me. Basically it is the trading that goes on out of the public eye. If Fidelity wanted out of HALO and Kirk wanted to support the share price, they could go thru the bid/ask process privately. Some estimates are that dark trading accounts for up to half of all trading in securities and that price discovery is possibly in jeopardy. It is a current debate I am trying to understand.
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