In article ,
Nicolaas Vroom writes:

Which is the best explanation that the observed galaxy rotation
curves (in general) do not match the calculated curves based on
visible (baryonic) matter?

Depends on whom you ask. MOND enthusiasts will say MOND, dark-matter
enthusiasts will say dark matter.

I think that the MOND people are right when they say that conventional
dark matter requires a very unnatural distribution in order to explain
the observations. For example, there is a very tight correlation
between luminosity and velocity dispersion, but the luminosity comes
only from stars while the velocity dispersion is influenced by
dark-matter halos orders of magnitude larger in size. On the other
hand, we shouldn't be surprised that we don't know all the contents of
the universe, and while phenomenologically it works well, it is
difficult to fit MOND in with the rest of physics. Also, even MOND
enthusiasts admit that dark matter works well on cosmological scales. I
think that dark-matter enthusiasts should investigate MOND phenomenology
more.

Recently, unconventional dark matter, which behaves like standard dark
matter in a cosmological context and MOND-like on galaxy scales looks
like a promising development. Look for papers by Justin Khoury.