At one time it was postulated the observed non-Keplerian rotational anomalies in galaxies were either caused by undetectable subatomic “Weakly Interacting Massive Particles”, or larger cold bodies (football-sized lumps of rock or ice?), MACHOs.

Galaxies appear to have their missing mass concentrated far past the visual boundaries, and no gas, dust or stars have been detected out there. This missing mass causes the galaxy to rotate more as a rigid body, not as a Newtonian orbital system with its mass concentrated near its geometrical center. Gas gives off microwave radiation, dust glows feebly in infrared (and obscures the light of distant galaxies) and stars give off visual light. Whatever this dark matter is, it appears to be perfectly transparent and does not emit any electromagnetic radiation. We can’t see it, except for its gravitational pull, hence the name.

My own guess is that there is no dark matter, that it is a tiny variation in Newton’s Laws (the exponent in the Inverse Square Law is not exactly 2) that is only noticeable over enormous distances. But that is only a guess. Still, its no more ad hoc than “dark matter”.