Why Is the Night Sky Dark? A Journey Through Olbers' Paradox from a Simple Question

"Why is the night sky dark if there are so many stars in the universe?"

Because you can't see distant stars. That's obvious!

...That was my reflexive thought, but apparently it's not such a simple problem. In fact, it seems this paradox remained unsolved for about 300 years from when it was first proposed until it was finally explained. To describe the paradox in more detail:

• If the universe is infinitely vast
• And stars are evenly distributed throughout it
• Then the Earth's view of the universe should be bright

In other words, "Earth is surrounded by infinite space filled with countless stars. Since Earth is illuminated by all kinds of light, the universe should appear dazzlingly bright from Earth's perspective." My intuition countered this with the idea that distant stars can't be seen. More precisely, "Light from distant sources disperses. Since distant light doesn't appear bright, of course the sky is dark." But this understanding is actually incorrect.

In reality, while the apparent size (brightness) of stars is inversely proportional to the square of their distance, the number of stars increases proportionally to the square of that distance. To rephrase, "Even if light weakens with distance, the number of observable stars increases inversely proportionally to that distance, so theoretically, the total amount of light Earth receives should remain constant." Wait, really?! But then... shouldn't the sky be blindingly bright?! Having understood this much, I finally reached the starting point of this paradox.

So why is it dark?

Some major premise must be missing or incorrect, or perhaps the calculation method is wrong.

The universe continues to expand, and light takes time to travel. The observable universe only contains objects whose light has had time to reach us since the universe formed 13.8 billion years ago. Therefore, while the universe may be infinite, the observable universe is finite—that's why it's dark. If the universe were infinite and we could observe light from the entire universe, the sky would be completely white. But since we can only observe 13.8 billion years' worth of the universe, it's dark.

Surprisingly, my initial answer, "Because you can't see distant stars," though lacking understanding, had accidentally arrived at the correct conclusion.