Isn't it the other way about? The reason we are hypothesising dark matter is because we can't explain some gravitational measurements. Did someone postulate dark matter particles, then later say 'well if these exist, they must cause some strange gravitational patterns.' ?
Yes, dark matter is of course an explanation invented because of observations. But it would just be a just-so story if you couldn't probe further consequences of that explanation by making new targeted observations.
