How could Voyager navigate inside the starless void? Especially since it was very large. Taking one year to cross. Some sort of inertial navigation maybe?
Voyager's navigational computer was able to calculate it's relative distance from Earth from the very first moment it was in the Delta Quadrant (70 thousand light-years). Not only that, but the amount of times Janeway told Paris to 're-set a course for home' (after having made a detour or three), makes it seem that such a distance could be plotted fairly quickly.
Their trip home was originally estimated at around 75 years. This was based on both the ship's speed, and the distance from home. Knowing this, one year's worth of 'Starless void' would seem like nothing for the navigational computer to plot a course through. All they would have to do is pick a point or two on the other side of the void, and constantly measure distance and heading from those points, to make sure they kept on a steady track.
This is largely explained in the episode. In addition to the ship's sensors (which Tom states are capable of detecting that the ship is at traveling at warp speed), Seven has completed an astrometric survey of the region and has noted a considerable number of features, including pockets of radiation. Even if the void was actual a 'void' you could navigate by those alone.
On top of that, we can ascertain from other episodes (notably TNG: Where Silence has Lease) that starships routinely use an "Inertial Guidance System" which evidently works extremely well, even in an environment where other sensors show no usable readings. It seems vanishingly unlikely that the crew of the Voyager wouldn't be able to cobble one of those together, even if the Voyager doesn't already come equipped with one.