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An additional contributing factor as to why the sky is dark at night is the fact that when light travels a great distance, it "stretches" with the expanding space, so the light's wavelength sometimes increases to a value outside the visible portion of the electromagnetic spectrum.
Today, at the beginning of the 21st century, we know so much more than we did even as little as 60 years ago. For example, we know that all of the naturally occurring elements up through iron (by atomic number) were created by fusion within stars, and that all the naturally occurring elements above iron were created in the cauldron of exploding stars. We also know that the Universe will not die in the conventional sense that it will contract again (to a single point), but in the unconventional sense that everything will ultimately have the same temperature.
There is still much to be learned about our cosmos, most notably that we do not understand the nature of over 95% of the energy-mass density of the Universe. Still, humankind persists in trying to master this curious and intriguing reality we live in.
The answer is that the Universe did have a beginning, some 13.7 billion years ago. At its birth moment, called the Big Bang, it expanded quite fast, and it continues to expand (in an accelerated fashion) today. As a result, light from some distant stars has not had enough time to get to Earth. Curiously, this (correct) explanation of Olber's Paradox was first offered not by a scientist, but by the American writer and poet, Edgar Allen Poe.
A good starting point for answering these questions is Olber's Paradox, which asks the seemingly ridiculous question "Why is the sky dark at night?" Apart from the obvious answer that the Sun is on the other side of the Earth, there is the profound objection that if the Universe is eternally old, there should be multiple sources of light along every line of sight, and the night sky should be ablaze with light. Since it is not, it begs Olber's question: why is the sky dark at night?
* Where did all the contents of the Universe, including all the elements, come from?
* How old is the Universe? Will it end, or go on forever?
* Did the Universe have a beginning? Will it have an end?
Cosmology, a branch of Astronomy, seeks answers to ultimate questions previously reserved for religious studies. Questions like:
Cosmology is the study of the evolution of the Universe, past, present and future.
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