SPY, which generally corresponds to the price and yield performance of the S&P 500 Index, one of the most popular indexes in the world, launched with just $6.5 million in assets. Today, it has more than $123 billion in assets under management (AUM), representing about 6.4% of ETF assets worldwide. As both the oldest and largest ETF in the world, SPY opened up a new way for investors to gain access to the markets.