In US equities this is routinely 40-50% of consolidated volume. The trades are real and reported, but they do not contribute a displayed quote before execution.
Almost all retail marketable flow is off-exchange, because wholesalers buy it through payment-for-order-flow and fill it internally, usually with some price-improvement versus the nbbo.
Example: you buy 100 shares with a quote of 10.00 / 10.02. An exchange fill costs 10.02. A wholesaler fills you at 10.0125, saving 0.75 cents, or 75 cents on the ticket, while capturing part of the spread as its own profit. The print shows on the trade-reporting-facility tape, not an exchange.
Related: internalisation, wholesaler, trade-reporting-facility, price-improvement, lit-market