Astronomers have determined the density and speed of dark matter in our corner of the universe. The finding helps bring dark matter out of the realm of the hypothetical and places scientists a few steps to closer figuring out what this invisible stuff that pervades the universe and holds galaxies together is made of. It also settles once and for all the question of which galaxy—our Milky Way or Andromeda—is more massive. The winner: The Milky Way.