And they made a killing off of this election. Those who bet on Biden to win made money, but that didn't hurt the bookies at all. That's because most of the bets came from Trump supporters, who were sure he would win.
Even crazier, a lot of those bets were made after it was known that Biden had won (but certification of the votes had yet to be done). Trump supporters believed he had actually won, and that the courts would overturn the election results.
Some bookies said it was like taking candy from a baby, and their profits were enormous.
You can read more about this here.