If you're looking for an under-the-radar stock that could deliver enormous gains in a defined time frame, the biopharmaceutical industry has you covered. Hardly a week goes by without clinical trial results or a regulatory decision that produces dramatic market movements.

Wall Street is pointing slightly lower in early trading but is on track to close the week with solid gains on healthy quarterly earnings reports from large U.S. corporations.

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.