The giant underbelly of the markets

Where does that come from when an equity trader sells a stock at a profit and makes money? The obvious answer is that it comes from whoever bought the stock. That's correct, but does that mean the person who bought the stock has lost money? No, at least not unless the buyer has had to sell at a loss. If the price of the stock rises, no one has made a loss. And given that over a period, the markets in general march upwards, equity investors make money on the aggregate. Everyone who invests in stocks understands all this unambiguously. When someone makes money in futures or options, then what happens? Where does that money come from? What about derivatives? Strangely, the answer to this question is less widely known a