Question

why light can travel with or without a medium

Answer 1

It is because wave-orientation of a light wave does not requires medium.Also,if we see particle nature of light,the wave is much stronh suvh that it does not requires medium.
-----------------------------Ayush

Answer 2

Electromagnetic waves (visible light is one) can travel with no medium because it has tiny electric and magnetic fields that vibrate instead any material vibrating (like how sound uses air, water, etc.) When light travels through a medium like air, electric and magnetic fields are the ones still vibrating, but the particles in the air actually slow down the light slightly.