That said, corporate America has been pretty much the only buyer of this rally that most everyone else—individual investors, pension funds, macro hedge funds—has sold.  There were $141 billion of stock buybacks authorized in April alone--a new record, according to Birinyi Associates. If the year-to-date pace keeps up, 2015 will record $1.2 trillion in total buybacks, handily outpacing the previous annual high of $863 billion set in 2007.  Indeed, share repurchases have become so commonplace that there are entire funds, like the TrimTabs Float Shrink fund, and the PowerShares Buyback Achievers Portfolio, tracking them as an investment strategy.  After all, the fewer shares outstanding, the better a company's earnings per share, the higher the valuation of each share—assuming the market valuation of the entire company doesn't change—and the more lucrative the stock options executives can exercise.  The central question for investors, then, is whether corporate America's appetite will wane as stock prices continue moving gradually to new highs.  Read MoreTarget announces share buyback after disclosure snafu  Brian Reynolds, chief market strategist at New Albion Partners, sees no sign that it will.  "There has never been a valuation level so high that CEOs as a group have stopped buying," he wrote in a recent client note.  "What has stopped them from buying has been the credit crisis of 2000 and the credit crisis of 2007," he added. At those market peaks, when companies were still buying, the price-to-earnings ratio of the S&P 500 was 29 and 25, respectively. Today, the market's "multiple," or premium paid per dollar of earnings, is a little under 19. That is based on Reynolds' formula of using two quarters of past and two quarters of estimated future earnings.  In other words, Reynolds said, stocks today are nowhere near "too high" for companies to stop buying.  Pressure may well come from elsewhere, whether shareholders concerned about the best use of corporate cash in the long term, or policymakers desperate for more investment in the economy. However, such pressure remains relatively low at the moment.  Meantime, other buyers could certainly step to the fore, such as individual investors who finally come back around to the market, or larger funds.  Yet it is precisely the reluctance of pension funds to enter the stock market that is helping to fuel its continued, buyback-led rally. It is also behind the worrisome rise in debt-fueled buybacks.  Public pension funds "don't want the perceived risk of stocks, yet they need to make 7.5 percent, so they keep pouring money into levered credit funds," said Reynolds. "That's why we're having such an intense credit boom."  And that "daisy chain" of credit flows is providing companies with the financing for buybacks that lift share prices.
Stocks have been weak during the last half-hour of trade. As it turns out, that's when the biggest buyers of this rally walk away.