Here's why:

* No interference: A vacuum is completely empty, meaning there are no particles for light to interact with. This allows light to travel in a straight line without being scattered or absorbed.

* Speed of light: The speed of light in a vacuum is the fastest possible speed in the universe, approximately 299,792,458 meters per second.

While light travels fastest in a vacuum, it can also travel through other mediums, but it will be slower and experience some scattering and absorption:

* Transparent materials: Light travels well through transparent materials like air, water, and glass. However, these materials slow the light down and can cause some scattering.

* Opaque materials: Light cannot travel through opaque materials like wood or metal. Instead, it is absorbed or reflected.

Let me know if you'd like to know more about how light interacts with different materials!