For example In a dark matter model galactic mergers are very frequent because the dark matter halos don't allow interacting galaxies to separate from one another through dynamical fiction. Yet the amount of massive elliptical galaxies is too small today.

I'm not sure where you're getting that from, but neither of those are correct statements. Dynamical friction is only fast in nearly equal mass mergers, which account for a small fraction of the mergers that occur. For example, here is a typical example of what the dark matter distribution is expected to resemble around a typical galaxy. All that substructure is from mergers that have not completed, i.e. where interacting galaxies have not merged together.

I have no idea where you're getting your claim that massive ellipticals are underabundant. Simple abundance matching between DM halos and observed galaxies reproduces many observed properties of galaxies across a wide mass range.

It is difficult to overstate just how precise the cosmic microwave background anisotropy measurements have become. A couple of decades ago, modified gravity models without dark matter could be made consistent with noisy CMB measurements, but it is a fact that dark matter is absolutely required to explain what is seen in the CMB. There are no serious cosmologists who have any doubt in the existence of dark matter. What remains uncertain is the nature of dark matter. Specifically, while particle dark matter (like WIMPs) are a popular candidate, we have no evidence that DM consists of particles. But there is now overwhelming evidence that dark matter does exist.