"The efficient market hypothesis (EMH) or theory states that share prices reflect all information. The EMH hypothesises that stocks trade at their fair market value on exchanges. A direct implication is that it is impossible to "beat the market" consistently on a risk-adjusted basis since market prices should only react to new information."
There are two sides – broadly, the academics and the active investors who vehemently disagree on this hypothesis. The former have indeed proved that most of the market participants can't beat the stock market and hence go on to say that market is efficient. Businessmen and thought leaders like John Bogle went a step further and built on this idea in his thesis on the topic of investment performance and went on to make a hugely successful venture Vanguard, which now manages about $6 trillion and counting on the basis of his thesis, thus validating it in a way that no one can really deny it. On the other hand, we have people like Benjamin Graham and his students like Warren E Buffett (one among many of his students but possibly the foremost) who made it their lifetime goal to explore every inefficiency in the market to create superlative returns over such a long period that it cannot be said that the market is efficient.