The primary thing we have detected is an attractive force within galaxies. Whether that’s otherwise-undetectable particles or a mistake in how we calculate gravity or something else, we definitely know it’s that there is more attractive force holding galaxies together than there should be based on detectable matter and general relativity.

Simply put: galaxies rotate too fast. Much, much too fast. That can’t be caused by repulsion between galaxies. Only by the stars within a galaxy being pulled towards the centre of that galaxy my than we would expect. Similar to how you have to spin faster to hold a big bucket of water horizontal without spilling than to hold a small bucket of water.