You are probably thinking of galaxy rotation curves which do not agree with theoretical predictions that assume only visible matter exists.

Gravity not following the ISL at large scales would indeed be a possible explanation for this. However, most alternative theories of gravity clash with other observations, such as the motion of binary pulsars or gravitational waves, whereas (unmodified) General Relativity is in excellent agreement. This is one of the reasons why the existence of dark matter is considered to be a much better explanation for the deviation of the rotation curves.

Yes, I was.

But I know so little physics that the distinction between dark matter and extra terms in gravitation is unclear.

It's pretty simple. You can use the known laws of gravitation and the visible distribution of matter in a galaxy to predict how fast stars at the edge of the galaxy should move. But when we do this we find that the stars are moving much faster than expected. So EITHER the formula for gravity is wrong OR there is extra matter there which we can't see, i.e. dark matter. Dark matter is currently the preferred explanation because it also explains a bunch of other, unrelated, phonenomena, but it has as yet never been directly detected or measured.