Exchange-traded funds were once an obscure niche of the mutual-fund industry. No longer.

For evidence, look no further than what's happened since the markets began recovering from the global financial crisis. Since March 2009, when U.S. stock markets hit their lows, investors have pumped $57 billion into ETFs holding U.S. stocks. U.S.-stock mutual funds over the same period have suffered withdrawals of $66 billion, according to Morningstar Inc.

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