Kalshi and Polymarket — the two biggest prediction markets — have become cultural phenomena over the past year. The sites allow users to wager on just about anything, from elections to Taylor Swift’s wedding to the winner of “Love Island.” This summer, they helped make the World Cup the largest gambling event ever.

Prediction markets allow users to gamble on future events through “event contracts,” where users place wagers based on a yes or no question. For example, a current bet on Kalshi asks whether the Federal Reserve will raise rates in September. The platforms make money by charging a fee for each bet. Other apps, including Robinhood, Fan Duel and Draft Kings, have moved into prediction markets, too. And Meta’s chief executive, Mark Zuckerberg, dispatched a small team to create a smartphone app similar to Polymarket and Kalshi.

In most parts of the world, the markets are legal, but it’s trickier in the United States. About 20 states have tried to bar them by enforcing anti-gambling laws, and Minnesota this year became the first state to pass a law to ban them. But Kalshi, Polymarket and the Commodity Futures Trading Commission argue that the platforms are commodity exchanges, not gambling sites. The two sides are battling it out in court.

The huge popularity of Kalshi and Polymarket has created fears about manipulation by those with insider information. Kalshi last week banned multiple users for insider trading, including George Santos, the former representative from New York.