Its a universal constant that emerges from the laws of physics. Central to relativity is the fact that the speed of light in a vacuum is an absolute constant. There is definitely space.
Since the vacuum is the same everywhere why would you even expect it to change.
According to the crucial postulate in Einsteins Special Theory of Relativity the speed of light is always constant relative to all freely-moving observers. However light actually slows down as it passes through different media. The speed of light in vacuum is a constant. In 1887 two particularly ingenious experimental physicists conducted an experiment to measure the speed of light changing as a consequence of the Earth travelling around the Sun.