About 40% of all U.S. stock trading happens in venues where you never see the order before it executes. They're called dark pools, and most investors have never heard of them.
A dark pool is a private trading venue — often run by a bank or a firm like Citadel Securities — where big institutions (mutual funds, pension funds, hedge funds) can buy or sell large blocks of stock without showing their order to the public market first. The trade only becomes visible after it's already done.
Insider Angle: if a pension fund needs to sell 2 million shares of a stock, doing that on a public exchange would show every trader in the world a giant sell order — and the price would crash before the fund finished selling, because everyone would sell first to get ahead of it. A dark pool lets that same trade happen quietly. But when 40% of volume is invisible until after the fact, the 'public' price you see on a stock chart is only reflecting part of the real supply and demand — fairness for large traders, less transparency for everyone else.
Try This: Pick any large-cap stock. Its options chain and short-interest data are real and current — compare the visible trading volume to how large its institutional 13F ownership is. A stock with heavy institutional ownership almost certainly has meaningful dark-pool activity you're not seeing in the daily volume figure.