Dark Matter

 Astrophysics for People in a Hurry by Neil deGrasse Tyson. Eighty-five percent of all the gravitational force in the universe comes from a source we cannot identify:

"...We've now been waiting nearly a century for somebody to tell us why the bulk of all the gravitational force that we've measured in the universe -- about eighty-five percent of it -- arises from substances that do not otherwise interact with 'our' matter or energy. Or maybe the excess gravity doesn't come from matter and energy at all, but ema­nates from some other conceptual thing. In any case, we are essentially clueless. We find ourselves no closer to an answer today than we were when this 'missing mass' problem was first fully analyzed in 1937 by the Swiss­-American astrophysicist Fritz Zwicky. ...

"Zwicky studied the movement of individ­ual galaxies within a titanic cluster of them, located far beyond the local stars of the Milky Way that trace out the constellation Coma Berenices (the 'hair of Berenice,' an Egyp­tian queen in antiquity). The Coma cluster, as we call it, is an isolated and richly popu­lated ensemble of galaxies about 300 million light-years from Earth. ...

"When we examine the Coma cluster, as Zwicky did during the 1930s, we find that its member galaxies are all moving more rapidly than the escape velocity for the cluster. The cluster should swiftly fly apart, leaving barely a trace of its beehive existence after just a few hundred million years had passed. But the cluster is more than ten billion years old, which is nearly as old as the uni­verse itself. And so was born what remains the longest-standing unsolved mystery in astrophysics.

3D map of the large-scale distribution of dark matter, reconstructed from measurements of weak gravitational lensing with the Hubble Space Telescope.

"Across the decades that followed Zwicky's work, other galaxy clusters revealed the same problem, so Coma could not be blamed for being peculiar. ... Per­haps the 'missing mass' needed to bind the Coma cluster's galaxies does exist, but in some unknown, invisible form. Today, we've settled on the moniker 'dark matter,' which makes no assertion that anything is missing, yet none­theless implies that some new kind of matter must exist, waiting to be discovered. ...

"Further research has revealed that the dark matter cannot consist of ordinary matter that happens to be under-luminous, or nonlumi­nous. ... [But] we have no clue what it is. It's kind of annoying. But we desperately need it in our calculations to arrive at an accurate description of the uni­verse. 

"Scientists are generally uncomfort­able whenever we must base our calculations on concepts we don't understand, but we'll do it if we have to. And dark matter is not our first rodeo. In the nineteenth century, for example, scientists measured the energy output of our Sun and showed its effect on our seasons and climate, long before anyone knew that thermonuclear fusion is respon­sible for that energy. At the time, the best ideas included the retrospectively laughable suggestion that the Sun was a burning lump of coal. Also in the nineteenth century, we observed stars, obtained their spectra, and classified them long before the twentieth­-century introduction of quantum physics, which gives us our understanding of how and why these spectra look the way they do. ...

"Having resisted attempts to detect it directly on Earth for three-quarters of a century, dark matter remains in play. Particle physicists are confident that dark matter consists of a ghostly class of undiscovered particles that interact with matter via gravity, but otherwise interact with matter or light only weakly or not at all.

"If you like gambling on physics, this option is a good bet. The world's largest particle accelerators are trying to manufacture dark matter particles amid the detritus of particle collisions. And specially designed laboratories buried deep underground are trying to detect dark matter particles passively, in case they wander in from space. An underground loca­tion naturally shields the facility from known cosmic particles that might trip the detectors as dark matter impostors.

"Although it all could be much ado about nothing, the idea of an elusive dark matter particle has good precedence. Neutrinos, for instance, were predicted and eventually dis­covered, even though they interact extremely weakly with ordinary matter. The copious flux of neutrinos from the Sun -- two neutrinos for every helium nucleus fused from hydrogen in the Sun's thermonuclear core -- exit the Sun unfazed by the Sun itself, travel through the vacuum of space at nearly the speed of light, then pass through Earth as though it does not exist. The tally: night and day, a hundred billion neutrinos from the Sun pass through each thumbnail patch of your body, every sec­ond, without a trace of interaction with your body's atoms. In spite of this elusivity, neutri­nos are nonetheless stoppable under special circumstances. And if you can stop a particle at all, you've detected it.

"Dark matter particles may reveal them­selves through similarly rare interactions, or, more amazingly, they might manifest via forces other than the strong nuclear force, weak nuclear force, and electromagnetism. These three, plus gravity, complete the fab four forces of the universe, mediating all interac­tions between and among all known particles. So the choices are clear. Either dark matter particles must wait for us to discover and to control a new force or class of forces through which their particles interact, or else dark mat­ter particles interact via normal forces, but with staggering weakness.

"So, dark matter's effects are real. We just don't know what it is."