It's a good idea to read up on the history of the efficient market hypothesis, and how Fama came to it. There's also different tiers with different layers of credibility.
Base tier (and the one pretty much everybody can agree on): The expected value of the error of the market is zero. This means that the market does not always fundamentally overvalue or undervalue assets, and that, over time, it will overvalue things as often as undervalue.
(... many intermediate forms ...)
Strong efficient market hypothesis: The market price always reflects all accessible information, and every market movement is driven by new information.
The strong efficient market hypothesis is pretty evidently wrong (the existence of all the various quant funds that are generating outsize returns is strong evidence; it would also mean there are no stock bubbles. Having lived through 2 bubbles in my life, I can tell you: Bubbles exist, and there are many forces outside of new information that lead to them inflating and popping).
