We can map it, weigh it, and simulate it, yet we still have no idea what it is. But dark matter is coming into the spotlight as never before.
Astronomers now know that for every gram worth of atoms in the universe, there are at least five times more of a new, invisible matter neither shining or blocking light.
We can also create model universes inside supercomputers that reproduce in stunning detail what we see around us in the night sky but only by assuming this invisible dark matter passes through us like a ghost.
Finally, in the past decade we have begun to almost routinely map out the invisible, finding it matching the simulation predictions.
Yet of the numerous candidates that particle physicists have thought up for dark matter, we are still far from knowing which is right—a quest that is every bit as grand and in some ways even more difficult than the search for the God Particle, the Higgs Boson.
First Clues
In 1978, Vera Rubin and Kent Ford discovered that stars in nearby galaxies were not moving as expected. Stars far from the center of a galaxy—where all the light (and hence mass) was concentrated—were moving far too quickly for the gravity of all the visible matter to hold onto them.
Therefore, some unseen source of matter must be providing the extra gravity to explain the stars in the outer reaches of the galaxy. The idea of dark matter had been brought into view, but next came the question, what actually was it?
Today, more than 40 years later, technological advances in astronomy have enabled astronomers to greatly expand on Vera Rubin’s pioneering work, and have found signs of dark matter in the motion of galaxies and stars everywhere. The motions of visible objects allow us to predict the dark matter distributions and these match the model universes created in supercomputers.
Such observations and simulations have lead to the staggering realization that the visible matter in our universe is the tip of the iceberg making up just 15.7 percent of all the mass.
But understanding how dark matter interacts with visible matter is only part of the puzzle. A crucial clue to the identity of the dark matter is whether it interacts with itself, and if so how strongly?
Not So Dark?
So how do you study the interactions of a particle you cannot see? The answer lies in gravitational lensing.
