Here's why:

* Speed of Light: The speed of light in a vacuum is a fundamental constant, denoted as 'c', approximately 299,792,458 meters per second.

* Interactions with Matter: When light travels through matter, it interacts with the atoms and molecules within that matter. These interactions cause the light to slow down.

* Density: The denser the material, the more interactions light has, and the slower it travels.

Therefore, a vacuum, having no matter to interact with, allows light to travel at its maximum speed.