Why don’t you calculate the light intensity? Think of the sun as a point light source like a light bulb. The intensity declines as the inverse square of the distance in all directions I.e. a sphere. The intensity is a rate of photons per square area. The fact the light drops off as a square of the distance the intensity drops off pretty quickly. However when I studied physics in college I learned experiments with the human eye indicated that humans when accustomed to extreme dark are able to see/detect an extremely low intensity of photons. But yes space is dark. Even with the countless numbers of suns the distances between bodies and other light sources makes space a generally dark place. The cosmic background radiation from the Big Bang is just under three degrees Kelvin (energy, which are photons) These are remnants of photons traveling around space left over from the Big Bang. However because there’s matter and light no matter how large the universe expands to there’ll always be some background light or energy. This is the wonderful thing about math. It can help you understand the universe and things like this question.