At 2,091, the S&P 500 (^GSPC) has gained a modest 2.3% since the beginning of the year.

For Goldman Sachs analysts, that’s about all investors are going to get.

“After an 11% plunge to start the year, US equities have rebounded by 14% and have now posted a 2% gain YTD,” Goldman’s David Kostin said in a new note to clients. “We maintain our forecast that S&P 500 will end the year at 2100, unchanged from the current level.”

For Kostin, the stock market faces three headwinds: “mediocre” GDP growth, a “mixed” earnings outlook, and high valuations.

Indeed, none of this gives traders and investors reason to pour money into the stock market. Citi’s Tobias Levkovich recently characterized the lack of catalysts as “the biggest issue” facing the bull market right now.

One force is keeping the stock market from selling off

In this market, Kostin expects households, foreign investors, and pension funds to be net sellers.

However, he also identifies buyers that should continue to bolster the market.

“Corporate repurchases are the main source of US equity demand,” he said. “We forecast S&P 500 gross buybacks will rise by 7% to $600 billion in 2016.”

Buybacks have been off to a slow start this year. Kostin estimates that corporations have spent $122 billion buying back stock through April 18, down from $257 billion during the same period a year ago. However, he also estimates that S&P 500 companies had around $210 billion of unused buyback authorizations in the pipeline. This is in addition to new buyback authorization to be announced as the year goes on.

With limited opportunities to invest in growth via capital expenditures, many companies have found that the best way to deploy excess cash is by returning it to shareholders through share buybacks and dividends.

Meanwhile, should the S&P 500 hang on to gains through Thursday, this bull market could become the second longest running bull market in history.

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Sam Ro is managing editor at Yahoo Finance

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