And when a former Goldman Sachs programmer was accused this month of stealing secret computer codes — software that a federal prosecutor said could “manipulate markets in unfair ways” — it only added to the mystery. Goldman acknowledges that it profits from high-frequency trading, but disputes that it has an unfair advantage.

Yet high-frequency specialists clearly have an edge over typical traders, let alone ordinary investors. The Securities and Exchange Commission says it is examining certain aspects of the strategy.

“This is where all the money is getting made,” said William H. Donaldson, former chairman and chief executive of the New York Stock Exchange and today an adviser to a big hedge fund. “If an individual investor doesn’t have the means to keep up, they’re at a huge disadvantage.”

For most of Wall Street’s history, stock trading was fairly straightforward: buyers and sellers gathered on exchange floors and dickered until they struck a deal. Then, in 1998, the Securities and Exchange Commission authorized electronic exchanges to compete with marketplaces like the New York Stock Exchange. The intent was to open markets to anyone with a desktop computer and a fresh idea.

But as new marketplaces have emerged, PCs have been unable to compete with Wall Street’s computers. Powerful algorithms — “algos,” in industry parlance — execute millions of orders a second and scan dozens of public and private marketplaces simultaneously. They can spot trends before other investors can blink, changing orders and strategies within milliseconds.

High-frequency traders often confound other investors by issuing and then canceling orders almost simultaneously. Loopholes in market rules give high-speed investors an early glance at how others are trading. And their computers can essentially bully slower investors into giving up profits — and then disappear before anyone even knows they were there.

High-frequency traders also benefit from competition among the various exchanges, which pay small fees that are often collected by the biggest and most active traders — typically a quarter of a cent per share to whoever arrives first. Those small payments, spread over millions of shares, help high-speed investors profit simply by trading enormous numbers of shares, even if they buy or sell at a modest loss.

The slower traders began issuing buy orders. But rather than being shown to all potential sellers at the same time, some of those orders were most likely routed to a collection of high-frequency traders for just 30 milliseconds — 0.03 seconds — in what are known as flash orders. While markets are supposed to ensure transparency by showing orders to everyone simultaneously, a loophole in regulations allows marketplaces like Nasdaq to show traders some orders ahead of everyone else in exchange for a fee.

In less than half a second, high-frequency traders gained a valuable insight: the hunger for Broadcom was growing. Their computers began buying up Broadcom shares and then reselling them to the slower investors at higher prices. The overall price of Broadcom began to rise.

Soon, thousands of orders began flooding the markets as high-frequency software went into high gear. Automatic programs began issuing and canceling tiny orders within milliseconds to determine how much the slower traders were willing to pay. The high-frequency computers quickly determined that some investors’ upper limit was $26.40. The price shot to $26.39, and high-frequency programs began offering to sell hundreds of thousands of shares.