> why would the laser beam would never stop passing by matter?
Because there is matter everywhere in our universe. It never stops.
> Because of the metric expansion of the universe?
It is true that, in order for there to be matter everywhere in our universe, it must be either expanding or contracting. (There is an edge case, the Einstein static universe, but it is unstable against small perturbations, like a pencil balanced on its point, so it is not a viable option.)
> Isn't it reasonable to assume that there is a skirt of the universe where matter keeps expanding into nothingness?
Because there is matter everywhere in our universe. It never stops.
> Because of the metric expansion of the universe?
It is true that, in order for there to be matter everywhere in our universe, it must be either expanding or contracting. (There is an edge case, the Einstein static universe, but it is unstable against small perturbations, like a pencil balanced on its point, so it is not a viable option.)
> Isn't it reasonable to assume that there is a skirt of the universe where matter keeps expanding into nothingness?
No. Such a model cannot match our observed data.