Wall Street is embracing its dark side.

As the stock market continues to climb, trading has increasingly migrated from established bourses like the New York Stock Exchange to private platforms, including dark pools, that are largely hidden from public view. The shift is helping big traders hide what they are doing in the markets, and regulators are worried that the development could obscure the true prices of stocks and scare away ordinary investors.

The movement, under way for several years, has gathered force recently. The portion of all stock trading taking place away from the public exchanges hit new highs over the last few weeks, amounting to close to 40 percent on several days, up from an average of 16 percent in 2008, according to Rosenblatt Securities.

The trend has bucked the government’s broad effort in recent years to move more of the financial industry out of the back rooms and into the light. The increasing opacity of stock trading in the United States, long the most transparent place in the financial world, is troubling for investors and regulators.

“We’ve been having a lot of discussions about whether we are reaching a tipping point between lit and unlit markets,” said Thomas Gira, head of market regulation at the Financial Industry Regulatory Authority, the industry-financed regulator.