As stocks rise, volume has been trending lighter. You can see the trends in volume declining from the March lows.

In fact, according to Bloomberg, S&P 500 volume for July is turning out to be one of the lowest volume months this decade.  This follows volume that was far below average in June.

Markets can, of course, rise on lighter volume.  Markets can do anything they want.  However, if volume is the oxygen of the bull, the market may soon be running short of breath.