No single.php file, so what is being used?

Hello, my theme does not have a single.php file, and one of the plugins that I would like to use requires I create a new file based on it.

Does anyone know what is being used if I don’t have a single.php?

I have an index.php, page.php, and a few others that don’t seem significant to this (sidebar, header/footer, css, etc)

The plugin that calls for the change is Members List Plugin.

Thank you!