I'll bite, what are the backrooms?
If I ever respond to your introduction posts, I refer to "the darker part of the uni campus". (I'm not the only one; Tony V does as well.) It's done in good fun, and sometimes, as in Persephone's case, they choose to run with it, something I'm more than happy to do.
But the backrooms, and just the dark part of campus, are (in RU's own shifting fiction) the abandoned areas of campus, where some less-than-savoury types permanently reside.