Why is the sky dark at night? Why isn't it bright as day with starlight shining in every single direction? This is an old and surprisingly complex question, and it took modern physics to answer it.* There are two interrelated reasons:
1. The universe is finite in both age and size. It began with the Big Bang about 14 billion years ago. It contains a limited number of stars, and since light takes time to travel, we can only see the ones that are less than 14 billion light years away. There just aren't enough stars in the part of the sky that we can see to fill it.
2. The universe is expanding fast in all directions, so everything is getting farther away from us. The farther away the receding source of the light is, the more stretched its wavelength is, and eventually the wavelength drops below our eyes' threshold to see the light. In fact, the sky is not dark. It reverberates with the energy from the early universe, just after the Big Bang, before matter coalesced, when it was very small, messy, and hard to understand. Special telescopes can detect these microwaves, but we can't.
Now, suppose we take this as a metaphor for life.
We are finite in time.
1. We were born. At first, we were small and messy.
2. We don't remember our own birth because the threshold of our memory doesn't go back that far. That's good, since it was probably unpleasant.
We are finite in space.
3. We can't observe everything. Knowledge is expanding in all directions faster than it can get to us. Facebook proves that.
4. We wouldn't understand everything anyway. Information can be stretched too thin to be intelligible. Think about how far TV commentators can stretch facts.
The same science that explains the Big Bang does not yet know if the universe will end with a Big Freeze, Big Rip, Big Crunch, Big Bounce, or something completely different although equally Big.
5. We don't know our own fate. That may be just as well, since it might not be especially entertaining.
6. Or maybe it will be. Cosmologist George Smoot, who won a Nobel Prize in Physics for his work to confirm the Big Bang Theory, made a special guest fanboy appearance on the Big Bang Theory. Scientists are great wags.
Our days are lit by one star, and the rest serve as little more than decoration in the night sky.
7. Half the time, we're in the dark.
8. However, the darkness is sublimely decorated, and nothing can thrill our imaginations like staring up at the sky at night.
A simple explanation.
An excellent short video.
The Wikipedia timeline of the prevailing theories of universe, starting with the Big Bang and speculating on our ultimate fate.
— Sue Burke