The Missing Universe
When you look up at the night sky, do you feel that something is missing? If you don’t, you should. Eighty-one years ago, astronomers discovered that much of the cosmos’ contents was invisible. They saw extra gravity pulling material around galactic centers, but they found no extra mass to account for that force. They deemed their conundrum the “missing mass problem” and came up with a partial solution: dark matter.
Dark matter, an invisible substance of unknown quality, character, and origin that permeates the universe, neither emits nor reflects light. But it makes up 85 percent of the cosmos’ mass. Or does it? The idea has some detractors, who say that the problem is not “missing mass” but a misunderstanding of gravity itself: Scientists need to modify Newton’s original ideas, and then the mathematical need to understand the universe via mysterious, elusive dark matter will disappear.
Above is a picture of the Whirlpool Galaxy (Messier 51). Like all other spiral galaxies, it seems to possess about 10 times more gravity than its visible contents can account for. Either dark matter surrounds these galaxies, or gravity olpe4rates differently than most scientists think.
As far as we know, matter exists in five (observable) states: plasma, gas, liquid, solid, or Bose-Einstein condensate. Quarks combine into protons and neutrons to form atomic nuclei held together by the strong force, and these can join with electrons into atoms held together by the electromagnetic force. A cloud of electrically charged atomic nuclei (ions) and electrons is a plasma. When atoms (or molecules) float loose, they form a gas. When atoms attach weakly to neighbors, they form matter’s rarest state in the universe, a liquid. When atoms rigidly bond to neighbors, they constitute a solid. At temperatures colder than any natural place in the universe, atoms can merge spookily into a single state, the Bose-Einstein condensate.
Dark matter would be a new form. Is there evidence? Perhaps. A galaxy’s rotation curve (compared above) compares the velocities of stars in the disk with their distances from the galaxy’s center. Observed curves appear flat, suggesting that a halo of unseen matter or gravity behaves differently in weak fields. Without dark matter or Modified Newtonian Dynamics, the curve would drop off at increasing distance.
Until dark matter particles can be confirmed by observation, astronomers can only keep gazing into the night sky, conducting experiments, and wondering whether most of the cosmos is really missing.
 See Bob Berman, “The Missing Universe,” Astronomy (42, 4, April 2014)