For most of the 217 years since its founding under a buttonwood tree on Wall Street, the New York Stock Exchange was the high temple of American capitalism

Behind its Greco-Roman facade, traders raised a Dante-esque din in their pursuit of the almighty dollar. Good times or bad, the daily melee on the cavernous trading floor made the Big Board the greatest marketplace for stocks in the world.

But now, even as the Dow Jones industrial average topped 10,000 for the first time since the financial crisis sent it tumbling, the exchange and its hometown face an unsettling truth: the Big Board, the symbolic heart of New York’s financial industry, is getting smaller.

Young, fast-moving rivals are splintering its public marketplace and creating private markets that, their critics say, give big banks and investment funds an edge over ordinary investors.

Some of the new trading venues — “dark pools,” the industry calls them — are all but invisible, even to regulators. These stealth markets enable sophisticated traders to buy and sell large blocks of stock in secrecy at lightning speed, a practice that has drawn scrutiny from the Securities and Exchange Commission.

These upstarts are utterly unlike the old-school Big Board, which is struggling to make its way as a for-profit corporation after centuries of ownership by its seat-holding members. Last year, its parent company, NYSE Euronext, lost $740 million.

While the exchange has been under assault since the beginning of the decade, its decline has accelerated in recent years as aggressive competitors have emerged. Today, 36 percent of daily trades in stocks that are listed on the New York Stock Exchange are actually executed on the exchange, down from about 75 percent nearly four years ago. The rest of are conducted elsewhere, on new electronic exchanges or through dark pools.

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