Wall Street Enters Darker Age With Most Stock Trading Hidden (1)

Here’s a surprising new fact about the world’s largest and most-liquid public equity market: Most of the activity on it isn’t public anymore.

For the first time on record, the majority of all trading in US stocks is now consistently occurring outside the country’s exchanges, according to data compiled by Bloomberg.

This off-exchange activity — which happens internally at major firms or in alternative platforms known as dark pools — is on course to account for a record 51.8% of traded volume in January. Barring an unexpected dip, it will be the fifth monthly record in a row, and the ...