This is an interesting question. Einstein once said that there is no aether since the Michelson-Morley experiment did not observe any change in the speed of light in any particular direction. However consider that the so-called free-space constants such as electrical permittivity are a known constant value, not zero and not infinite. Maxwell was able to construct the speed of light c from such constants and derived it using his well known Maxwell's equations. Then along came quantum theory and the uncertainty principle which stated that even the vacuum is not "empty", many so-called virtual particles are constantly being created and reabsorbed into the quantum vacuum in a tiny amount of time called the Planck time. Energy conservation is briefly disobeyed. So space itself even when free of atoms or molecules or even photons is not empty in the true sense of the word. This means that in some sense it must be a kind of medium although not in the ordinary sense of the word as used in say materials physics where the medium is structured by atoms or molecules in some kind of order e.g. crystals in the case of metals. Indeed Einstein also states how free space possesses a kind of stiffness which makes it hard to warp, e.g. gravitational waves are waves in this medium, space-time.

Q1. There is no measurable difference between the speed of light observed in planetary or galactic space and that measured in a vacuum in a terrestrial laboratory.

(with little more than a telescope and a stopwatch Delambre got to within 1% accuracy, and that was 200 years ago!)

Q2: There is nothing forbidden about transluminal speeds (if you don't mind imaginary mass). But Special Relativity forbids relative motion *at* the speed of light. Perhaps you have not heard about tachyons?

I think "c" the light velocity is affected by many parameters and forces. That it may be changed little bit in far space but our tools could not recognize that change now.

Can you give us your definition of "c", so we can compare?

c- never find s true at any instance in space or lab ; the value is altering fad , hence traveled photons have massless quantum energy always fully charged until ists reflected or refracted d or bent or send thro... space a perfect medium than n our earthly particles as dark energy and dark matter ALWAYS matters to be considered ----research,,,,@@@@@

>How different from "c" is the light velocity in space?

No different at all, by definition c is the velocity of light in vacuum measured to be constant according to the Michelson-Morley experiment which negated the existence of a medium into which the light would propagate with different speeds depending on relative motion. Einstein used this powerful negative result as a principle to build the special theory of relativity.

> wondered if it would be possible for a particle to travel faster than light in the space.

From the homogeneity and isotropy of space together the principle of relativity (equivalence of Galilean systems) together with the principle of constancy of the speed of light the Lorentz transformations can be derived, one of the transformations mandate how speeds must be composed never getting beyond the limit c.

>Can the space be considered a medium?

That was the view previous to relativity the search for aether resulted always negative and the speed of light found always constant then it was realized that no medium was needed for the fields to exist and propagate.