<i>It is very hard for me to state my questions logically...</i><br /><br />And trying to answer relativity questions is also very difficult, for several reasons:<br /><br />1. The concepts are usually counter-intuitive.<br /><br />2. You have to be precise with your wording (while also keeping it simple, to the point and accurate).<br /><br />3. The maths are formidable.<br /><br />4. The classical Einsteinian model of the universe (Theory of Relativity and Special Relativity) is not universally accepted (although it has repeatedly passed muster through experiment or observation).<br /><br />But attempting to formulate cogent questions and answers does force one to think in a clear and logical manner and this will ultimately improve your overall communication skills.<br /><br /><i>...and therefore, could that mass also affect the speed of light through the medium and perhaps the speed of light is not a constant outside of our galaxy(ies) because it is not in a vacuum?</i><br /><br />Let's take the case of the photon, which travel's at "c" velocities (the speed of light). Since it's travelling at "c", it does not experience the passage of time, from its perspective; although from our relative perspective we observe some photons to be billions of years old!<br /><br />Since the photon is both created an annihilated at the same instant, from its perspective, there is no "Time " available for "m" (mass) to effect a photon's velocity.<br /><br />Now let's look at "m". Mass is static -- all the "m" (and "E", energy) in the Universe was created during the Big Bang event and it does not change <sup>(1.)</sup>. The expanding space and time in the universe must curve around all the static mass and the manifestation of this conflict is observed as gravity. Gravity is simply the curvature of space and time as it expands around static mass. <br /><br />So, the mass in the universe cannot effect the velocity of the photon or "c", but it does cause space and time itself to be curved.<br /><br />