Saying something is "just a theory" does not make any sense, scientifically speaking. Scientific theories provide explanations that have been tested and proven.
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The law of gravity describes and quantifies the attraction between two objects. But the law of gravity doesn’t explain what gravity is or why it might work in this way. That’s because that kind of explanation falls into the realm of theory.
The theory behind this legend goes like this: Daytime is bright, due to the sun. If you view a sunless piece of sky through a long tunnel, there should be no way for the sunlight to get into your eyes. Therefore, you should see stars.