Speed of Light in a Vacuum
To be precise, what we usually call the "speed of light" is the speed
of light in a vacuum (the absence of matter). In reality, the speed of light
depends on the material through which the light moves. For example, light
moves
slower in glass than in air and in both cases the speed is less than in a
vacuum.
However, the density of matter between the stars is sufficiently low that the
actual speed of light through most of interstellar space is essentially the
speed it would have through a vacuum, so we don't introduce much error by ignoring
the difference.