Open this photo in gallery:Dmitrii Musku/iStockPhoto / Getty ImagesPassive investing has made investing cheaper and more accessible. Its success can also encourage a comforting assumption that buying the market removes the need to think about what is being bought. With a handful of companies dominating the S&P 500, that assumption deserves scrutiny.There is a strong case for passive investing: low fees, broad market exposure and a long record of many active managers underperforming their benchmarks. These are real advantages.But a low-cost fund can still own expensive stocks, and a long list of holdings can conceal considerable concentration.Portfolio manager Michael Green,…