[RightWhale]

Do they know for sure that the speed of light is the same in space than in atmospheric conditions?

Light is slower in the atmosphere and even slower in water or glass. But the assumption that light moves at the speed of light in the vacuum is very major and is still an assumption. The theory of relativity plays with this concept a lot, so much that you could say the speed of light in a vacuum is not in general the speed of light in a vacuum. You can certainly not make the jump to the idea that nothing can go faster than the speed of light.

[PSYCHO-FREEP]

In Einstein's time, most Scientists agreed that space was a vacuum. But now, there are strong indications that there are dark matter particles driving the accelerated expansion of the Universe. In a vacuum, the Universe would expand at a constant rate, also referred to by Einstein as the "Cosmological Constant". But due to indications in "Red Shift" the Universe is accelerating.