Wikipedia defines the protagonist of a story as follows:
A protagonist (from Ancient Greek πρωταγωνιστής, prōtagōnistḗs 'one who plays the first part, chief actor') is the main character of a story.
The protagonist is at the center of the story, makes the key decisions, and experiences the consequences of those decisions. The protagonist is the primary agent propelling the story forward, and is often the character who faces the most significant obstacles. If a story contains a subplot, or is a narrative made up of several stories, then each subplot may have its own protagonist.
The protagonist is the character whose fate is most closely followed by the reader or audience, and who is opposed by the antagonist. The antagonist will provide obstacles and complications and create conflicts that test the protagonist, and revealing the strengths and weaknesses of the protagonist's character.
To me, the last section of the description reads to me as if a protagonist "needs" an antagonist, but my gut feeling tells me that that's not always the case. In "slice-of-life" stories, there are clearly protagonists and side characters, but no clear antagonists.
As such, I would like to know if a story necessitates the existence of an antagonist for a protagonist to exist.