What happens to energy?

Our sun and source of all almost all of our energy We learn in physics that matter and energy can neither be created nor destroyed – aka the First Law of Thermodynamics. Of course, matter can be converted to energy, as a tiny, tiny bit of matter gets destroyed in the fusion reactions happening inside our sun, with a huge resulting release of energy. Hence, life can exist on our planet 93 million miles away.

Here’s a wild question, unencumbered by the thought process… what happens to the energy from the sun as it continues to travel and travel and travel? We know it travels great distances, as evidenced by light from very distant galaxies. It spreads out, of course, so is weaker and weaker (per cm2), but is there any possibility that some of that energy might slow down (not as in a lab experiment, but just in free open space)? And if light energy goes slower than the speed of light, would it not change in its nature? Are there any conditions where energy will turn into matter? Could this be the source of dark matter? It’s just starlight that turned back into matter? At the distances where this would happen, there would be tiny concentrations of energy left, so a very tiny amount of matter would result, as even a single atom of matter would create great amounts of energy, so weak star light would make matter that is only tiny fractions of atoms, probably too small to have much effect on any other light, hence, seemingly dark, or more like invisible.

I doubt this is a novel idea, but I have no time or interest in researching other’s thoughts on the matter, and it is just a musing for me.