“The comedian George Carlin used to do a sketch where he was a kind of hippy weather man. “And the weather tonight is … dark, man.”
But he didn’t ask the question, why is it dark? Once you get past the flippant answer “duh, because it’s night-time”, you see why it is an interesting question. If there are an infinite number of stars, shouldn’t the sky be bright?”
Since the sky isn’t very bright at night – a lot of dark plus a few pinpoints of light – this is called Olber’s Paradox. Put simply, if the universe is infinite then wherever you look you should see so many stars that the night would be brighter than the day.
Johannes Kepler considered this question but he argued that the universe must be finite. Otherwise the total flux from all the stars would make the night sky “as luminous as the sun.”
Suppose we gaze out in any direction from Earth, imagining a thin sphere of radius R around us. Unlike Kepler, Newton’s model assumed a uniform, infinite (and static, or not expanding) universe, the number of stars in the shell is proportional to R2 and the intensity of radiation from the shell reaching Earth is proportional to 1/R2. So, according to Newton’s model such shells stretch to infinity so the sky can never be dark.
Of course, now we know that the Universe is expanding – it has a beginning – and stars and galaxies aren’t tastefully arranged in neat little spheres.
Lots of bits are dark and the combined effect is to make the night sky dark.
Furthermore, there’s more ‘dark’ than ‘light’ and the Universe expansion probably isn’t uniform. Best estimates suggest that if we calculate the energy needed to overcome gravity, dark energy ( the stuff that accelerates the expansion of the Universe) makes up roughly 68% of it. Dark matter makes up another 27%, leaving the “normal” or baryonic matter that we are familiar with to make up less than 5% of the cosmos.