[yobbo]

No, since expected intraday gains are negative, this would be a losing strategy. Closing price seems to be lower than opening price on average.

The suggested explanation is: buy orders are placed before open, raising opening price. The gains on the holdings are then larger than the cost of buying in the opening, and selling during the day.

[ksdale]

The market goes down during the day, that isn't the same thing as everyone losing money during the day. On net, all of the market participants lose money, but this is still consistent with sophisticated financial firms making money off of trading with big actors who are less price sensitive and retail traders.