The truth is that antagonists are not always villains, villains are rarely the only antagonists, and while not every story has a villain, almost every story has at least one antagonist.
Right now you might be thinking, "Whaaa-???" and scratching your head.
Allow me to explain.
Many sources, including the Merriam-Webster dictionary, simplify the relationship between hero and villain, protagonist and antagonist. The villain/antagonist is the one who opposes the hero/protagonist.
This is most clearly seen in archetypal children's stories of Good vs Evil. Good Red Riding Hood vs Big Bad Wolf. Innocent Hansel and Gretel vs Bad Cannibalistic Witch. The opposition of good hero against evil villain is black and white.
But very little, if anything, is really that simple.
What about an opponent who isn't evil? Here I'm thinking of a story such as The Fugitive (the 1993 movie version with Harrison Ford and Tommy Lee Jones).
In The Fugitive, [*Spoiler alert!] Dr. Richard Kimble is falsely convicted of killing his wife. When Kimble escapes on his way to prison, U.S. Marshal Samuel Gerard is brought in to capture him.
As far as Gerard is concerned, Kimble is guilty and is literally an escaped convict. Gerard is on the side of justice and is doing his job. In short, he's not a villain.