It depends on the canon, but the way I envision it is that there are multiple dimensions which exist as clusters, like all the dimension where earth exists being one cluster. The backrooms is its own cluster, its the cluster of dimensions of things which dont exist, which never existed, which are discarded and in a way deleted. This dimensional cluster of Unbeing has only recently begun colliding with our own cluster, and we are slowly merging with it. The Wanderers who get noclipped into the Backrooms are only the beginning. No one knows when, but in time, the entire reality will be noclipped into the backrooms and warped to merge with the rest of this infinite expanse.