Every child is afraid of the darkness. For them its not simply the absence of light, its the absence of information. Even if you know that there should be a wall or a doorway right in front of you, how can you be sure without seeing it? What other dangers might lurk there, besides stubbing your toe on a chair, and what was that movement you just saw out of the corner of your eye?
But what if all this wasn't just imagination, What if the darkness itself could take physical shape?
I started exploring this idea, where in a dark space "things" would manifest over time, similar to how fog condenses into dew, except that this dew can move, has some simple instinct-based intelligence and mild hostility towards sources of light an heat. When exposed to light (electromagnetic radiation with wavelengths < 1000 nm) the creatures would quickly disintegrate, but something the size of a wolf - which could materialize during a long cloudy winter night in an open field - would last long enough against a torch or small flashlight to cause some serious harm to the wielder.
To make sure things don't get out of hand, the maximum size of creatures is based on the geometry of the space. A half empty tetrapak of milk would simply spoil faster, a drawer is big enough for a bunch of fruitflies, the space under the bed or in a wardrobe is ideal for bugs or a couple of mice, a cellar with broken lighting could generate something cat- or dog-sized and deep caves can house huge horrors.
I ran into problems with explaining the process behind this natural phenomenon, specifically the conservation of energy. The creatures need to have at least some mass to be dangerous or they could be defeated with a ventilator, but generating mass from nothing is hard. Bonding together surrounding air molecules could work, and with 1 $m^3$ air weighting roughly 1kg, it corresponds well with the creature sizes i listed. But when the creatures are made from highly compressed air, every sunrise would be announced by a devastating wave of explosions, which turns the original design upside down by making light more dangerous than darkness.
Question: By which scienc-y process could the described creatures spawn from dark spaces, while using as little magic and handwaving as possible?