Do you close your doors completely? Do you leave drawers almost but not quite shut? Do you leave the thinnest line of darkness seeping through the crack of an unclosed portal? Do you wonder what may be there?
Many people have a fear of un-shut doors. The doors must be all the way closed or all the way open. To them, that slight bit of incompleteness is unacceptable. As person who has repaired many door hinges and drawer slides, I prefer that things are totally shut. But that's not what I'm talking about. I'm talking about the irrational fear that something is there.
In optics, a slit is used to diffract (bend) light in order to spread a small bit over a wide area. There are those who believe the same happens to reality when there is a small slit such as a slightly open door. That thin crack, they think, is enough to let the slightest bit of some dark dimension spread into ours. Or, just as bad, that bits of ours gets bent into elsewhere.
Imagine what ancient things may lurk on the other side of that crack. Do they hunger? What would you look like to them? Would they see you as a threat or possibly a meal? Would they reach out with strange appendages to draw you in? Would they come out to get you? Would they stay on their side of the crack and stare at you?
What about our side being bent into the other? How many times have you sat something down to have it be gone a moment later? Where did it go? Could it have been pulled by the diffraction of our world into another? What other things could be pulled in; your car keys, a pet, a child? When will you be walking by the un-shut closet door and find yourself somewhere else? Will you be able to get back?
Of course, in optics it is not only a slit that gets used. Sometimes a razor edge is used because it is the edge that really matters. There are other edges. If a gap in a cabinet door could bend reality, what about the edge of a bed? You've probably never been comfortable with what's under your bed. Do you give thought to other places? What goes on near your feet under your desk? What visions are blocked out by your keyboard tray?
These are just musings, of course, on the irrational thoughts of some individuals. I'm sure you never give thought to mysterious things watching you from an opened door. You don't go around concerned that the slight opening in a woman's purse my harbor a fiend in the darkness. It's unlikely that you think the pressure of your clothing that tickles your leg or back is any kind of eldritch monster reaching to drag you back to die in its realm.
No, it's just the door to the closet. The clothes that were there when the closet was open and the lights were on are the only things there now. You don't have anything to fear from that breach, no matter how dark it may seem or how much it feels like something is watching you. You go right ahead and leave that door or drawer almost but not quite closed.