These so-called alternative trading systems are propagating rapidly, are often labeled "dark pools" because of their nebulous and murky nature. Estimated to handle about 1 out 10 shares traded each day in the U.S., dark pools are meeting a need by institutions to grab or dump stocks quietly -- and anonymously. Like Liquidnet, many of them sport imaginative brand names in a nod to science fiction, such as Sigma X, VortEx and Block Alert.
In the harsh light of a public marketplace like the floor of the NYSE, an institution trying to pull off a massive trade runs the risk of making a big splash that will move the market. But in a dark pool, a big fish can jump in without so much as a ripple.