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Internet is often transmitted through a series of light-filled tubes called fibre optics, which are obviously
light. This is dangerous because others can see the light, exposing the data.
Darknet is the secret networks run over copper, ethernet, satellite or wireless connections, which cannot be seen due to the fact they aren't made of light.
This makes it harder to monitor them, or for google to index them because they can't use a scanner or camera to search through the data.