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If the sky is bright on earth at daytime, why is it dark in space even if there is the Sun and other stars? Why is it dark in space all the time? What happens to the light emitted by the stars?

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This question forces us to define some terms and to understand what we mean by “light.”  The universe is flooded by light, if by that term we mean energy in the form of light waves/particles traveling through “empty” space. If we use the term light as meaning those wave frequencies visible to Man, then “daylight” on Earth is caused by reflection and refraction of light waves in the atmosphere and off surfaces of non-transparent (by definition) matter, and by direct perception of the rays from the sun.  When we can “see” the moon, for example, it is because the light rays of the sun are reflecting off its surface. Darkness is a human term for the absence of humanly visible light waves.  Cats, for example, are not “in the dark” at night, because their eyes can perceive different and fainter light waves.  Bats are “in the dark” in caves, but can navigate through other sense organs.  Light is a form of energy; daylight is a human term.  A parallel can be found in your car radio or your iPhone.  The “signals” (actually another form of light) are always there, but you can only perceive them when your “receiver” is on and transforming them into sound.  The same is true of the term "dark in space."

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