> If we didn't have those cosmological problems that need explaining, no one would be suggesting dark matter.
The thing is, the dark matter hypothesis explains more than just certain cosmological problems. It also explains the rotation curves of galaxies, gravitational lensing by galaxy clusters, and the velocity dispersions of galaxies, galaxy groups, and galaxy clusters.
Dark matter is not an ad hoc hypothesis to solve a single problem; rather, it's an ad hoc hypothesis (though still more plausible than any alternative) that explains a large number of observed phenomena over many different length scales.
The thing is, the dark matter hypothesis explains more than just certain cosmological problems. It also explains the rotation curves of galaxies, gravitational lensing by galaxy clusters, and the velocity dispersions of galaxies, galaxy groups, and galaxy clusters.
Dark matter is not an ad hoc hypothesis to solve a single problem; rather, it's an ad hoc hypothesis (though still more plausible than any alternative) that explains a large number of observed phenomena over many different length scales.