The efficient market hypothesis supports the idea of buying index funds.

an index fund, as its name implies, is a fund product that takes a specific index, such as the Shanghai and Shenzhen 3 Index, the S&P 5 Index, the Nasdaq 1 Index and the Nikkei 225 Index, as the target index, and takes the constituent stocks of the index as the investment object, and constructs a portfolio by purchasing all or part of the constituent stocks of the index to track the performance of the target index.

Generally speaking, index funds aim at reducing the tracking error, making the trend of portfolio change consistent with the underlying index, so as to obtain roughly the same rate of return as the underlying index.