Light can be slowed by particles and transparent mediums like air, water, and glass. The ratio by which light is slowed is called the refractive index of the medium, this value is always greater than one. Outer Space is considered to be a near-perfect vacuum. The refractive index of outer space is generally considered to be one, which means that light isn't slowed down. Therefore, light can be considered to be travelling at the speed of light with outer space as the medium.
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