There are cracks forming under the surface of the stock market

While the US stock market has shown remarkable resilience lately, if you look closer, there are some cracks forming under the surface. Business Insider senior markets reporter Joe Ciolli discusses a measure called dispersion, which shows the degree to which the market's best performers are diverging from the worst. It's high right now, which some market experts think shows rising investor worry and risk aversion. This doesn't necessarily represent an immediate threat to the eight-year bull market, but it's something traders should keep an eye on if the situation worsens.