What is the .wellknown folder in File Manager?

This folder appeared in my public_html folder, dated a week ago. Inside is an empty folder called acme-challenge, which sounds like something out of a Road Runner cartoon. Can anyone tell me what this folder is and what might have created it? Having been hacked before, I’m very cautious about files just appearing.

We have an integration with Let’s Encrypt to provide free SSL certs. For the longest time they still had to be done manually but about 2 months ago we rolled out automatic SSL so that all sites would be provisioned with certificates automatically as long as they were eligible (this runs on a cron job and is tiered based on the number of domains it has to cycle through). The way Let’s Encrypt ensures you’re the domain owner is they load a small file from the web so the integration created that folder to house the file. Unfortunately it can be confusing because the integration doesn’t remove the folder afterwards, however I can assure you it’s harmless and not a hack.