This milestone marks the fifth consecutive month of record highs for dark pool trading and the third month in a row where hidden trades exceed half of all trading volume[1]. The shift towards off-exchange trading represents a significant change in market structure, with dark pools evolving from their original purpose of handling large block trades to now executing smaller orders. The average size of dark pool transactions has decreased dramatically, from 430 shares in 2009 to approximately 200 shares in recent years[2]. This trend highlights the growing prevalence of dark pools in everyday trading activities, moving beyond their initial role of catering exclusively to institutional investors.

Citations:
[1] Wall Street Enters Darker Age With Most Stock Trading Hidden https://finance.yahoo.com/news/wall-street-enters-darker-age-110003632.html
[2] Deep Dive into Dark Pool Trading - How they might accidentally be ... https://www.reddit.com/r/Superstonk/comments/mv5kbm/deep_dive_into_dark_pool_trading_how_they_might/

I saw this story elsewhere and one of the explanatory comments made was that this is entirely driven by penny stocks and that for larger stocks the percentage is virtually unchanged. Sorry don't have a link or supporting proof.