In almost every building, there is one corner, one small enclosure that no one ever looks at. It's the corner in the basement that has been blocked by a disused sofa for years; the thin space in the attic between the wall and the stacks and stacks of crates full of junk you never use, but could never throw away. The space that never sees the light of day, or any other kind of light at all. Where darkness does not merely dominate, but practically oozes out from around the edges of its prison.
No one knows quite how long a space must remain concealed for it to acquire this particular property, nor if there are any specific conditions it must meet. But it is a far more common occurrence than you might think.
In newer buildings, when this happens, the residents often report feeling cold when passing by, even in attics during the hottest of summers. Whenever contemplating taking a quick peek to see if there is anything actually there, an unnatural dread seizes them, and they leave the room quickly, if not quite running. Once left behind, the feeling passes, and it is quickly forgotten, or laughed off.