Good thought, and actually radiation density is a major factor in cosmological models! Sadly it’s nowhere near sufficient to explain the effects observed by dark matter, as well as a few other properties it’s been deduced to have.

Hahaha what? MOND (which is what you’re describing) was heavily looked into, it’s just that it doesn’t seem to work well.

Physicists aren’t some dark cabal trying to cling to theories of the past, everyone wants to find something new. Taking a human view of things, if MOND was a good approach people wouldn’t jump on it, the chance of blowing apart GR and writing your name in history?