In addition, because researchers use the positions of stars to indirectly place the locations of these HVCs, “the distances of most clouds are either unknown or only poorly constrained, so we have a rough idea where they are, but we don’t know exactly.”
Westmeier explained that, because of these problems, it’s “impossible to reconstruct a trajectory, which is part of the reason for why the nature of most of these clouds has remained a mystery for so many decades.”
This means we can't be sure where they started, or where they'll ultimately end up.
There is one exception to this. Some particularly ginormous HVCs are being jettisoned from the Large and Small Magellanic Clouds – satellite galaxies to our own – right through the Galactic South Pole of the Milky Way. The HVCs here are so expansive, clocking in at tens of thousands of light-years in length, that they’re collectively referred to as the Magellanic Stream.
Although they were discovered back in 1965, and linked back to the Clouds in 1974, they’ve only now been painstakingly mapped in relation to the other HVCs in the galaxy. In this case, it’s quite clear where they’re originating from, but their form and structure cannot be entirely explained at present.
In any case, HVCs are unlikely to be passing objects. They contain an awful lot of (mostly electrically neutral) hydrogen, which hypothetically means that they could be contributing to star formation if they travel to the right places.