Carl Gauss is considered by some as the most brilliant mathematician who ever lived. He is sometimes called called the "Prince of Mathematicians." I even named one of my computers after him. One of his contributions is the Gaussian Distribution where real valued random variables tend to cluster around a single mean variable. Does that sound like prices dancing around during an average trading day, only to settle on a single value when the market closes? Moments ago I checked the data on Vanguard's Total Stock Market ETF. The price was $59.36 with a high of $60.61 and a low of $59.15. While the price may close on the low or high of the day, most likely the price will be somewhere in between. No one has an idea what will happen to the price of any investment during any given trading day and that is what we mean when we talk about efficient markets.
"What distinguishes index funds is that they don't presume to have greater wisdom than the collective market, but instead try to channel its wisdom to your advantage." – Zack O'Malley Greenburg, a financial journalist.