A simple back-of-the-envelope estimate shows that it takes an -extremely- small amount of very low-density 'invisible' matter - assuming that it's spread out ov

A simple back-of-the-envelope estimate shows that it takes an -extremely- small amount of very low-density 'invisible' matter - assuming that it's spread out over the huge 3D 'spherical' volume of a galaxy - to make up the missing mass. On the order of a few -atoms- per cc. Given all of the unknowns about interstellar/ intergalactic matter, including the density of EMF, it's not too hard to understand why the account sheet won't balance. It's a hard and wonderful problem.