Galactic motions aren’t consistent with the amount of observable matter. So either the basic theory is wrong or there is a lot of undetectable matter and energy in the universe. Since theory can’t ever be wrong there must be invisible, undetectable matter that affects everything.
Think about that for a minute. LOL. Most of the universe is made up of undetecable, invisible matter and energy. You have to believe this because they all have so many college degrees.
My unscientific theory is that physics and the rules of nature are much different at the macro level, just like they are on the micro level. Quantum physics on massive steroids.