Over the years there have probably been different explanations for kryptonian powers, but the only one I can recall is the high gravity thing which is very outdated.
I want to read about how people have explained both kryptonian powers and what kryptonians know of those powers and why they don't travel to another galaxy.
I've always felt that the best explanation is that at some point in time kryptonians used technology to change their physiology and that gave them powers, and for generations they were able to fly on Krypton. Eventually a war broke out and one side decided to infect their yellow sun and turn it red so that they could deescalate the conflict, and the reason why kryptonians didn't travel that much to other planets is that their interstellar technology wasn't very developed specifically because they had all those powers.