The blade of a metal cutter is shorter than that of the scissors used by tailor. why

It depends on the amount of pressure needed to cut your material.  Metal is a tougher material to cut though, so the blades must be shorter to create more pressure to break through the metal.  Cloth on the other hand is easier to cut through so the blades can be longer in order to cut more in each snip.

3. dark matter

empty space could also be a simple vacuum