- gravitational lensing measurements

- velocities in galaxy clusters

- getting the fine details of the complicated, wiggly cosmic microwave background spectrum right to extreme precision

- getting the fine details of cosmological structure formation (as measured from matter density fluctuations on very large scales) right

- galaxy collisions, like the Bullet cluster (hard to explain with modified gravity)

- the fact that some galaxies have much less dark matter than others (very hard to explain with modified gravity)