What type of wave does not require a medium to travel through?

Electromagnetic waves are unique among wave types because they do not require a medium for propagation. This means they can travel through a vacuum—like the vacuum of space—where there is no material substance. This characteristic is due to the way electromagnetic waves are formed and propagate. They consist of oscillating electric and magnetic fields that support each other and move through space independently.

In contrast, mechanical waves, such as sound waves and water waves, require a medium (solid, liquid, or gas) to travel. Surface waves require the interaction of the surface of a medium to propagate, while longitudinal waves, a subset of mechanical waves, also depend on a medium—these move through compressions and rarefactions in the material. Therefore, electromagnetic waves stand out as the only type capable of existing and moving through the empty spaces of the universe.