Dark pools let institutions trade large blocks without showing their hand on the public order-book. Trades are reported after the fact to the consolidated tape, so they do appear in time-and-sales, just not in advance.
Roughly 40% of US equity volume executes off-exchange. Retail commentary often treats dark-pool prints as secret signals; they are mostly ordinary institutional business and do not reliably predict direction.
Example: a 500,000-share block prints at $101.15 with no visible order beforehand. It was matched in a dark pool at a price near the public midpoint.
Related: order-book, liquidity, time-and-sales, market-maker