The approach of the dark matter cloud would be seen in perturbations of orbits and its effects predicted long before they occur.
Consider --- if you use traditional physics to completely calculate the orbit of all the planets around the solar system, everything falls out pretty much perfectly --- except for the orbit of Mercury --- it is elliptical and gradually rotates around the sun:
- ~5,030 arc seconds of this rotation are from the gravity of the sun
- ~530 arc seconds from the other planets
- ~43 arc seconds cannot be accounted for by the classic laws of physics, but are perfectly explained by general relativity
