Here's why:

* The Speed of Light: Light travels at a constant speed in a vacuum, which is approximately 299,792,458 meters per second (often rounded to 300,000,000 m/s). This is denoted as "c".

* Interaction with Matter: When light passes through any material, it interacts with the atoms and molecules within that material. This interaction slows the light down.

* Index of Refraction: The degree to which a material slows down light is measured by its "index of refraction." A vacuum has an index of refraction of 1, meaning light travels through it at its fastest possible speed. Other materials have higher indices of refraction, indicating slower light speeds.

Therefore, while light can pass through many materials, it travels fastest in a vacuum where there is no matter to interact with.