how far away are we from developing a metal that has no mass?
Such a material as the Roswell T-bars would be the holy grail in material science. However, to quote you "...practically no weight..." is the key phrase. Though theoretical physics (string theory) predicts the existence of takions (I think that is the correct spelling), or massless subatomic particles, I think that massless metals made from massless particles is not in nature's playbook.
As for interdimensional travel, you do it everyday when you walk across a room, or up stairs, or just sit and do nothing. You experience interdimensional travel in the four known dimensions (x,y,z,t). Now if this isn't your cup of dimensional tea, theoretical physics (string theory once again) predicts the existence of from ten to twenty-six dimensions. But these dimensions are said to be so tiny (billion billions times smaller than an atom) that they cannot be observed. But sorry to disappoint once again, but if true, your atoms are already peforming interdimensional travel.