A required template file in WordPress themes is index.php.
The idea is that this is the theme’s last line of defense, the template file that’ll be used if no other ile its. In other words, if the theme doesn’t have a category.php file, and WordPress wants to render a category archive, then the system will look for others.
First it’ll check for archive.php, the fallback for archives, then it’ll go back to index.php, the final fallback. Treat index.php that way, and make sure it returns something.