We now know that the universe is filled with dark matter. This strange substance cannot be seen, heard, felt or touched, and doesn't interact in any way with ordinary matter. Even so, its presence can be felt by its gravitational influence. It is the enormous amount of dark matter that causes galaxies to form and to spin as rapidly as they do.
While dark matter may ultimately be beneficial to the cosmos, "dark pools" in the financial markets doesn't seem like a good idea. When large investors buy large blocks of stocks outside of public view, they do so to obtain a tactical advantage. The market effect of dark trading is that the real value of openly traded stocks is less certain. This is another example of how the playing field is tilted away from mom and pop investors and towards the rich and powerful.