Hey so for a while ive noticed there is this effect with the eye ive not been able to describe it properly so not really been able google the pehnominon hoping here people know

So it goes like when im in a place or room of dim nature like say there is no light source but a small candle light in the distance and your in a bathroom with no bulb when i leave my eyes open and unfocuses like as if you were day dreaming everything begins to go pitch black like darkness creeps in from the borders of my eyes to the center like a natural vignette until i can see anything in that barely visible dim room and then when i blink it returns back to normal

Anyone know what this is?