Actual supply and actual demand are unknown. Expected supply and expected demand determine price. As new information is obtained, supply and demand expectations change. As expectations change, bid and offer prices are changed. The efficient market theory says that prices reflect all available information.
New information causes buyers and sellers to change their bids and offers.
Since new information causes prices to change and no one knows what the new information will be, exact price changes cannot be predicted. What makes tomorrow's price different from today's price is new information. People or companies that acquire information before anyone else in the market may earn a profit in the market. Companies pay millions of dollars to obtain supply and demand information first. These companies use this information to set offer and bid prices.