If it was a problem with the large size of galaxies, the error would be proportional to the size of the galaxy. Calculations show that ordinary matter consistently makes up around 1/5 of the matter needed to account for the gravitation. That is an error factor of 400%. To somehow assume that just because mass is gathered in something we call galaxies it suddenly follows completely different equations that consistently gives the same error, is much more unreasonable than the existence of dark matter imo.
Also, there are other independent proofs that result in almost exactly same amount of dark matter.