As the stock market continues to climb, trading increasingly has 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 the development could obscure the true prices of stocks and scare away ordinary investors.

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

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.

In March, Australia introduced new rules to limit trading off-exchange, following the lead of Canada, which put regulations in place last fall. In the United States, the Securities and Exchange Commission so far has declined to act.

Dark pools, like public exchanges, give investors a place to connect with buyers and sellers of stock, but the pools are subject to less-stringent regulations than public exchanges.

Often run by big banks, dark pools do not require buyers and sellers to publicly announce their intention to trade stocks, allowing traders and investors to hide behind a veil that only the operator of the pool can penetrate.

That appeals to a pension fund that wants to buy a million shares of Ford stock, for instance, because it allows the fund to avoid tipping off competitors who could push the price of the stock up.

But the biggest factor pushing trading away from the public exchanges is the ongoing decline in volatility in stock prices, traders say. When share prices are rising or falling sharply, investors want to quickly and reliably get their trade done, leading to a preference for the safety of an exchange. In calmer trading, on the other hand, the anonymity of dark pools is more attractive. What’s more, dark pools generally are cheaper to use than an exchange.

“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.