Magnetic lines of force are the imaginary lines in a magnetic field which show us the path along which a single notrh pole will move. They can never intersect. Otherwise at the junction point, the north pole will have two directions to move simultaenously. How can any particle move along two paths at a time?

Isolated single monopoles do not exist. We always get a pair of north and south pole only. The lines of force start from a north pole and end at a south pole. Hence magnetic lines of force are always closed curves. They are crowded where the field is strong and are less and far from each other where the field is weak. Far away from the poles of a magnet, the field is very weak. Hence over there they won't tend to crowd, but remain away from each other.

They can easily pass through vacuum.