Light travels through a vacuum. Light travels at its greatest possible speed when moving through a vacuum. This is because there are no particles with which it can interact, which lengthens the distance light has to travel as it is absorbed and re-emitted by the particles.
The speed of light is a universal constant in a hypothetical vacuum. A perfect vacuum is not possible in reality because there are always particles present. Quantum physics reveals that virtual particles flash in and out of existence as a result of Heisenberg's uncertainty principle, which states that one can never know the exact position or speed of an object.