The Bloomberg Barclays US Aggregate Bond Index, or the Agg, is a broad base, market capitalization-weighted bond market index representing intermediate term investment grade bonds In addition to investment grade corporate debt, the index tracks government debt, mortgage-backed securities (MBS) and closely as possible, before fees and expenses, the total return of an index that measures the performance of the intermediate-term U.S. corporate bond market. An intermediate-term bond is one that matures not in the short-term or the long- term but in lot of different types of bonds: government, corporate investment- grade, corporate high-yield You may find a bond index fund that's even less costly.