How Zwicky already ruled out modified gravity theories without dark
  matter
  Various theories, such as MOND, MOG, Emergent Gravity and $f(R)$ theories
avoid dark matter by assuming a change in General Relativity and/or in Newton's
law. Galactic rotation curves are typically described well. Here the
application to galaxy clusters is considered, focussed on the good lensing and
X-ray data for A1689. As a start, the no-dark-matter case is confirmed to work
badly: the need for dark matter starts near the cluster centre, where Newton's
law is still supposed to be valid. This leads to the conundrum discovered by
Zwicky, which is likely only solvable in his way, namely by assuming additional
(dark) matter. Neutrinos with eV masses serve well without altering the
successes in (dwarf) galaxies.
