Prediction markets just had their best two days ever, generating at least $5.83 billion in trading volume on the first Saturday and Sunday of the National Football League season, according to Aldrin Research.

Ninety-one percent of that volume was tied to sports and combo trading, the prediction-market version of a parlay bet at a sportsbook, and $4.89 billion of the trading took place on Kalshi.

Sunday trading data from Polymarket's U.S. platform wasn't yet available, but analysts at Jefferies estimate it saw $404 million in trading volume on Sunday, below its daily average of $532 million during the World Cup.

Polymarket has a data sharing partnership with Dow Jones, the publisher of Barron's.

Kalshi's lead position looks unshakeable. The race for second is more of a tossup, with DraftKings making up the most ground recently. Its weekly prediction-market trading volume more than doubled for two weeks in a row leading into this Sunday.

This early in the season, prediction markets are mostly focused on onboarding new bettors. Barron's reported that in the week leading up to NFL Sunday, the majority of promotions advertised on social media were sign-up bonuses.

The sixteen prediction markets in competition with one another flooded the airwaves last week. Those advertising wars have become personal, with DraftKings running ads that denigrated the user experience on Kalshi. Neither firm responded to requests for comment on the advertising.

Fees from trading volume aren't the only way sportsbooks like DraftKings can make money from prediction markets. Another route is market making on rival prediction-market platforms.

Most market making happens on combo trades, where multiple individual bets are linked into one trade that pays off if each prediction proves correct. Unlike at a traditional sportsbook, where the proverbial "house" sets the odds on those bets, prediction markets put users up against another customer who sees value in the other side of the trade. Market making is when an individual or trading team takes the other side of large numbers of combo bets.

Barron's reported last week that FanDuel and multiple other online sports betting operators are market making on Kalshi. Flutter, FanDuel's parent company, said in its second-quarter earnings report that it expects to generate $50 million in market-making revenue in the second half of this year.