I've noticed that in a lot of maps for human migration, there's usually a really unspecific arrow pointing from the Iran area that goes through Kazakhstan, Mongolia, and Russia to North America. These maps all seem to have this dark area with little info, why's that?
You mean the dark area is central and eastern Siberia? If so, the reason for that is, that we have not found any prehistoric human fossils yet there. Siberia is very scarcely populated, so it is not very likely that someone is finding any fossils there.
Anyway, the route is just a theory and also the only plausible route for explaining human beings in the Americas (1), since Bering strait was a landmass in the past and the only landmass which connected Asia and the Americas.
(1) Another theory for example is that human beings did not went Out-of-Africa but also "emerged" in the Americas simultaneously. There are also a lot other theories.