I don’t think this an official word, but rather one that was invented and has become well-used. The Dictionary of Obscure Sorrows defines it as:
The eerie, forlorn atmosphere of a place that’s usually bustling with people but is now abandoned and quiet – a school hallway in the evening, an unlit office on a weekend, vacant fairgrounds […]
Merriam-Webster has no listing for it. Neither does Wiktionary.
A lot of images of “kenopsia” seem to be of abandoned locations, which might not be the original meaning. The original kenopsiatic environment seems to be a place which is normally very busy, but is momentarily quiet, like Grand Central Station in the middle of the night, for example.
Why I Looked It Up
There’s a subreddit called /r/kenopsia that’s filled with pictures that convey this feeling.