As seen above, Apple makes up 3.8% in the S&P and even more in QQQ, somewhere around 15%. Below is Apple's performance last few weeks:
That huge spike since mid January is what has been keeping the stock indices from being flat or down. Its a bit unusual to say the least that one company has such a big impact on the overall market.
As Bespoke Invest notes:
While it may seem as if the market has been doing well lately, it really hasn't done anything over the past ten trading days. Since February 3rd (a Friday), the S&P 500 is flat, and the Dow is actually down about 100 points. While Apple -- with its huge weighting in the S&P 500 and Nasdaq -- has really helped buoy the market over this time period, underlying breadth has weakened, and the VIX fear index has shot up 25%.So will be interesting to see how the market reacts as today's negative reversal in Apple drove the market drop to a significant degree.