Its Dark Materials

Each galaxy lives within its own three-dimensional halo of dark matter whose gravitational field corrals the stars within it. Without the stars, the halo would still be there, albeit invisible to our eyes; but if the halo vanished, the stars would scatter into the depths of the universe – just as a Christmas tree remains a tree with or without the pretty lights. Whereas without the tree, the lights would merely be a puddle of colour on the snowy ground.