"It doesn't come as a shock to me. When you compare it to the last bull market, it's really not low," said Ryan Detrick, senior technical strategist at Schaeffer's Investment Research. "A lot of people clearly haven't been in the market. If they have been, the money's been in gold and bonds, kind of ignoring stocks."
Stock investors have missed quite a party.
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Since the March 2009 lows, below-average days for volume yielded market gains of 296 percent, while above-average days saw losses of 37 percent, according to Bespoke Investment Group.
"People can complain all they want about volume, but who wouldn't prefer gains over volume?" Bespoke's Paul Hickey said in an analysis.
Research from Schaeffer's confirms the idea. The firm studied dollar volume, or the combination of price and trading activity, and found that the current level is below the 2007 stock market peak and subsequent selloff but well above the 2003-06 recovery rally.
Indeed, June was the market's only losing month this year and also its best for volume.
Trends in July suggest that investors are getting more involved, as they have poured about $37 billion into equity funds, according to data research firm TrimTabs. Most of that money—$29.3 billion—went into exchange-traded funds.
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Historically speaking, this is now the fifth-strongest bull market ever.
The S&P 500 has gained 148.5 percent in its 1,592-day run. The next target is the 1942-46 gain of 157.7 percent.
The 13-year bull market from 1987 to 2000 saw equities gain 582 percent. While this market has a ways to go before hitting that milestone, doing so likely won't be dependent on how many people come along for the ride.
"Throughout the entire bull market, people have been carping on the fact that volume has been weaker than average, and therefore the gains we have seen are somehow illegitimate," Hickey said. "You can fret all you want about volume...but if that is the main criteria you apply, you have missed out on a historic move in the U.S. equity market."
—By CNBC's Jeff Cox. Follow him @JeffCoxCNBCcom on Twitter.