Many stock market participants believe share prices are close to a peak, and this may be a piece of evidence to back their view.

U.S. stock funds have suffered a $44 billion outflow so far in 2015, the worst start for a year since 2009, according to a report from Bank of America Merrill Lynch (BAML) obtained by MarketWatch.

And what caused the move? Weaker-than-expected economic data, the dollar’s upward surge and overly bullish sentiment, according to the BAML strategists.

Earnings reports need to improve for stock fund inflows to increase. "Restoration of EPS [earnings-per-share] confidence" is "needed for US inflows."

As for the economy, growth slowed to 2.2 percent in the fourth quarter from 5 percent in the third quarter, and many analysts think it will shrink further this quarter. When it comes to the dollar, it has hit multi-year highs against a range of currencies in recent weeks.

Many investors are concerned about stock valuations. Robert Shiller's cyclically adjusted price-earnings ratio for the S&P 500, which includes 10 years of earnings, stands at 27.3, topped only by 1929, 2000 and 2007. Those, of course, were periods that preceded market crashes.

Meanwhile, investors are enthusiastic about Europe's lower valuations. European stock funds have seen a $46.6 billion inflow so far this year.

One expert who isn't bearish on U.S. stocks is Jeremy Siegel, professor of finance at the University of Pennsylvania.

He predicted at the beginning of 2015 that the Dow Jones Industrial Average would hit 20,000 by year-end, and he's sticking with that forecast.

"I think we're in a range-bound market now, maybe 17,000 to 18,500. But I still believe 20,000 is the fair market value of the Dow given interest rates," he told CNBC.

"It's not going to be there in the next month or two. Maybe by the end of the year if things go well. We're in a choppy market [now]."

"We need to see the dollar stabilize, maybe even sink down a little bit" to boost earnings, giving stocks impetus to rise, Siegel said. Numerous companies have reported that the dollar's strength — it has risen to multi-year highs against a range of currencies in recent weeks — is biting into their earnings.

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