Containing the 500 largest publicly-traded U.S. companies by market cap, the S&P 500 is arguably the best indicator of the performance of the overall U.S. stock market. At least, it has been in the past.

But something is happening that's making the index's performance somewhat misleading. As of March 6, the S&P 500 has gained 7.0% year to date. But 6.5 percentage points of that gain are due to the performance of just five companies.

You may be wondering how this is possible, given the index is so massive. Are the other 495 companies really putting up mediocre results?

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Source Fool.com