The central issue of efficient markets concerns:

The central issue of efficient markets concerns the degree to which market prices reflect all available and relevant information. If markets are efficient, then all information is already incorporated into prices, and so there is no way to "beat" the market because there are no undervalued or overvalued securities available. This is known as the Efficient Market Hypothesis (EMH), which states that it is impossible to outperform the overall market through expert stock selection or market timing, and that the only way one can possibly achieve higher returns is by purchasing riskier investments.