What travels alone but is never alone, has a name but doesn't exist

Why this works

This riddle plays with the dual meanings of "travels" and "exists." A shadow appears to move alongside a person (it "travels alone"), yet it is always connected to the object or person casting it, making it "never alone." The phrase "has a name but doesn’t exist" refers to the fact that we call it a "shadow," giving it a name, but a shadow is not a tangible object—it can't exist independently from the light and object that create it.