The question "why is it dark at night" was asked by scholars many centuries ago. However, they could not find the answer. Although the question seems relatively straightforward, the answer to it is not easy. At least not without the knowledge that science brought in the early 20th century. So why is it dark at night?
The first thing that comes to mind is that it is dark because the sun is not shining. In fact, the daylight we see is caused by the Earth's atmosphere. The sky is dark on the moon, even though the sun is shining in the sky.
Furthermore, we can assume that the universe is infinite and that we will always see a star in every direction we look. However, we know that the universe is not infinite for our needs, i.e. it is not infinite in the time plane for us as observers. We know that the universe originated about 13.8 billion years ago. But the universe is big enough to be everywhere we look.
But if we look very far away, we find that the light from such distant objects appears red. If we look even further we have to look in the infrared spectrum. This phenomenon is known as the so-called redshift, which is a manifestation of the well-known Doppler effect, which we observe (specifically hear) every day when, for example, a car passes us. The reason for the redshift is the expansion of the universe.
The reason for the dark night sky is therefore our ability to observe only visible light and the expanding universe, which "robs" us of this visible light.