The Vanguard S&P 500 ETF (VOO) has reached $1.03 trillion in assets, making it the first ETF to surpass the trillion-dollar mark. VOO charges an expense ratio of 3 basis points (0.03%), significantly lower than the SPDR S&P 500 ETF Trust (SPY)'s 9.45 basis points (0.0945%). This fee difference, along with VOO's open-end fund structure allowing immediate dividend reinvestment (unlike SPY's unit investment trust structure which causes cash drag), contributes to its growing market preference. While SPY maintains liquidity advantages for traders, VOO offers long-term holders a more cost-effective option for S&P 500 exposure.