The oldest exchange-traded fund listed in the U.S. has just been surpassed in assets under management by a Vanguard ETF that more cheaply tracks the S&P 500 index.

Retail investors are getting into the idea of passive investing and we’ve seen a steady increase in the funds poured into ETFs. A top choice for investors, Vanguard has set a gold standard for low-cost funds that are ideal for long-term investors.

The Vanguard S&P 500 ETF (VOO) has claimed the top spot for the ETF with the most assets, passing SPDR S&P 500 ETF Trust (SPY). Vanguard Head of Bank and Institutional Janel Jackson says VOO's growth is aided by model portfolios.

Vanguard Group's Standard & Poor's 500 ETF overtook rival State Street Global Advisors' SPDR S&P 500 Trust, on Tuesday to become the world's largest exchange-traded fund, according to data from FactSet.