You're looking at arguably one of the best maps of dark matter that we have. Astronomers tinted the dark matter concentrations in the giant galaxy cluster Abell 1689 blue. They figured out the location of those concentrations by using gravitational lensing. Image courtesy NASA , ESA, and D. Coe (NASA JPL/Caltech and STScI)

In the 1978 follow-up album to "Born to Run," Bruce Springsteen uses darkness on the edge of town as a metaphor for the desolate unknown we all face as we grow up and try to understand the world.

Cosmologists working to decipher the origin and fate of the universe must identify completely with The Boss' sense of tragic yearning. These stargazing scientists have been facing their own darkness on the edge of town (or on the edge of galaxies) for a long time as they try to explain one of astronomy's greatest mysteries. It's known as dark matter, which is itself a placeholder – like the x or y used in algebra class – for something unknown and heretofore unseen. One day, it will enjoy a new name, but today we're stuck with the temporary label and its connotations of shadowy uncertainty.



Just because scientists don't know what to call dark matter doesn't mean they don't know anything about it. They know, for example, that dark matter behaves differently than "normal" matter, such as galaxies, stars, planets, asteroids and all of the living and nonliving things on Earth. Astronomers classify all of this stuff as baryonic matter, and they know its most fundamental unit is the atom, which itself is composed of even smaller subatomic particles, such as protons, neutrons and electrons.

Unlike baryonic matter, dark matter neither emits nor absorbs light or other forms of electromagnetic energy. Astronomers know it exists because something in the universe is exerting significant gravitational forces on things we can see. When they measure the effects of this gravity, scientists estimate that dark matter adds up to 23 percent of the universe. Baryonic matter accounts for just 4.6 percent. And another cosmic mystery known as dark energy makes up the rest – a whopping 72 percent [source: NASA/WMAP]!

So what is dark matter? Where did it come from? Where is it now? How do scientists study the stuff when they can't see it? And what do they hope to gain by solving the puzzle? Is dark matter the secret to solidifying the standard model of particle physics, or will it fundamentally alter how we view and understand the world around us? So many questions to be answered. We'll start at the beginning – next.