Given how many gazillions of stars there are, in every direction one may look, it’s not irrational to wonder why the night environment is dark. The conundrum may seem worse if one is familiar with Black-body radiation, how light is emitted from every object, with the mean wavelength varying monotonically with the temperature of the object.
There are several factors, in the cause of this darkness, the first being that our eyes, our retinae, and neural processing in the brain of stimuli from the eyes, were evolutionarily selected for a particular environment, the savannah in East Africa, and, apparently, for day-time activity. They do not process light outside a relatively narrow band of wavelengths of light, or below a threshold of amplitude, and are damaged by light above some threshold of strength.
Another factor is that the number of stars are indefinite, not infinite, while the surface of our viewing hemisphere is infinitely divisible. Coupled with the varying distances of stars, this results in differences in the amplitudes, and even continuity, of the light that reaches us, from each star. A confounding factor in that continuity is the varying refraction in our Atmosphere, through which the light passes, causing that twinkling effect.
But, vying for most important in the apparent phenomenon of darkness with the function of our optical sensing systems is the absorption of light by dust in interstellar space. In accordance with the Law of Conservation of Energy, the energy of the light absorbed is not stored in that dust forever, but is re-emitted, mostly as light in lower energies, lower wavelengths, in the Infrared band, which most, if not all, humans can not process in our optical systems.
So, the truth is there is no such thing as “darkness”, only our inability to see what light there is, under particular conditions.