Like @kspaans says it's more about buying a representation of the market i.e. passive investing. Though, in the case of the S&P500, 500 large cap US stocks is h

Like @kspaans says it's more about buying a representation of the market i.e. passive investing. Though, in the case of the S&P500, 500 large cap US stocks is hardly buying the market, so a keen investor would try to create a truly diversified portfolio including domestic, international, equities, bonds, real estate, commodities, etc.