We got a little idea of why the speed of light if constant and that it really is based on the discussion on relative mass concept and experiments done, respectively but, How does it happen? Is really unanswered.

I merrily progressed to unit 23 and found myself floundering in confusion - I had lost the internalisation of the constant nature of the speed of light! So I'm back here again (and doing other reading) to get a better grip on the concept, not just an acceptance of the 'reality'. I figured that if one were to whack a rock or lead ball with a bat, it would shoot off at a measurable speed. If the ball were replaced with a golf ball, it would move faster. If the ball had zero mass and in a vacuum, just a light blow would cause it to shoot off really fast - if it were of truly zero mass then the speed should be infinitely fast!? So why is the zero rest mass photon restricted to the finite speed of light? Energy and Mass are related so perhaps the energy of the photon gives rise to pseudo-mass??

Binary stars give us a very strong evidence about the speed of light, awesome!

You are treating the photon as a sort of very small golfball and that is where the analogy breaks down. The energy transfer is not governed by a momentum conservation (which would indeed have infinite speed for a particle with no mass) but should be more considered like plucking a guitar string. The speed of the wave on the string is governed by the tension of the string not by the plucking force. It is the "tension" of space that determines the speed of the photon wave.

Interstellar and planetary science may be different. Accumulations of dark matter seem to work in single galaxies. As a medium travelled through, dark matter can slow light. What we all don't consider is the Japanese tests on the self propagation of water. Polar outflow of water from planets of water may influence planetary speeds of light. And dark matter.