Have speed of light tests ever been performed in space ?

How can we be 100% sure that light speed is a constant throughout the universe filled with dark matter and dark energy and a ton of stuff we know nothing about?”

Yes, numerous tests regarding the speed of light have been performed in a variety of areas. The speed of Light History, How is the speed of light measured?

The speed of light has two issues, one we understand well, c the speed of light, the other is v, the velocity of light; ergo v/c.

What causes this function of v/c, is exactly what you question “How can we be 100% sure that light speed is a constant throughout the universe filled with… a ton of stuff we know nothing about?”

Yet, we do know about the ton of stuff, baryonic matter, and, we know Gravity affects light. Einstein told us so in his 1905 paper: On the Electrodynamics of Moving Bodies See section #10

Simply explained the distance light travels in a straight line, a vacuum, is different than the distance light travels in a gravity field, “the universe filled with… a ton of stuff”. The above employs Einstein’s R-curvature Equation describing the function of v/c and the energy involved to move a photon the height of the curvature arc. Clearly light travels farther in a curved path than a straight path.

Now for the good part: “How can we be 100% sure that light speed is a constant throughout”? Excellent question. And there is a prevailing thought that there is no change, meaning, that because the Universe is homogeneous and isotropic, there is no change or that v=c.

I take issue with that conclusion. Why?

In effect by claiming v=c and that there is no change into and out of the gravity field (a gravity well), is to claim there is no effect of gravity on the photon except on the x and y-axis, which is called gravitational lensing. Again, I take issue with that.

I claim we should proceed further, and include the z-axis in the gravitational lensing accounting of gravity’s effect on light, to correctly account for the v/c slight difference.

And, it is quantified in the above diagram as the amount of energy it takes to move a photon the height of the arc. Merely a simple derivative of E=mc^2. This helps us analyze gravity’s effects on light 3d instead of just 2d gravitational lensing on the x and y-axis.

Why is this important? First, it helps us more fully understand what light has to go through to get to us, to observe. Next, by understanding the gravitational effects on the z-axis, distance, and how it is observed, as cosmological redshift, then we know to account for it correctly.

When we account for cosmological redshift correctly we find the Universe is not ‘expanding at an increasing rate’ requiring dark energy (an unknown).

We advance and find the Universe is contracting at an increasing rate, requiring no dark energy, and, in compliance with known physics and observation of baryonic matter and spiral galaxies contracting everywhere, ToE Gravity & Light paper #G aka “the universe filled with… a ton of stuff”