Bottom line is that while money supply stays the same in an economy, all the money is not actually in the stock market, say the Nikkei. It is invested elsewhere or is in the bank or the sofa cushions etc. Thus while overall wealth/money does not fall, the wealth in the market certainly can fall when there are gaps down with no trading and no one has shorted. In that instance, there is no one within the confines of the particular market who has taken money out of he market to move elsewhere. The same does not hold true in futures markets where contracts are created by someone going long and someone going short. Gaps in those markets do create a transfer of money/wealth within the particular market and can be moved to other investments. While it is very mathematically true that total money in an economy doesn't change , the total wealth of a given market can go down with no money removed to be used elsewhere unless there are shorters.