How far away is the nearest dark matter?

Look no farther than the tip of your nose. Dark matter isn't just "out there" -- in the deepest reaches of the cosmos. Dark matter should be, in theory, all around us. We simply can't detect it.

Dark matter is the mysterious substance thought to pervade the Universe. The total mass-energy budget of the Universe is broken down this way: about 70% dark energy, 25% dark matter, and 5% ordinary matter.

Dark energy is the name given to the force that appears to be speeding up the expansion of the Universe, sort of like a repulsive force, a counter to gravity. (And scientists have no idea what dark energy is. The question will likely still be around in ten years, when our readers enter college and graduate school.) Dark matter is the name given to the substance that seems to be the glue holding galaxies together. We cannot see dark matter; it doesn't radiate or reflect light like the atoms that make up paper, people and planets. Instead, dark matter reveals itself through gravity.

For example, think of all the stars rotating around a spiral galaxy. Gravity from the stars in the galaxy core keeps all the stars in the spiral arms from flying off. This is similar to how the Sun's gravity keeps the Earth in orbit. When scientists add up all of the visible mass in a galaxy, however, they quickly realize that there isn't enough mass to keep those outer stars in orbit. Some unseen mass -- the dark matter -- must be the source of gravity needed to keep the galaxy intact.

While scientists don't have a clue yet as to what dark energy is, they are close to understanding the nature of dark matter. A few facts appear to be very clear: Gravity is a result of mass warping space, sort of like a bowling ball on a mattress, curving the mattress. Less-massive objects are attracted to more-massive objects as if they are rolling down hill. Because dark matter contributes to at least 80% of the mass of the Universe, ordinary matter will be attracted to it. It rolls right in. Wherever there is ordinary matter, there is a sea of dark matter with it.

Furthermore, ordinary matter often heats up to high temperatures as it falls into the gravitational well created by dark matter. Telescopes that detect X rays from hot gas in clusters of galaxies are essentially mapping regions of underlying invisible dark matter.

There are several theories that describe the nature of dark matter. One theorized type of dark matter -- the leading candidate, in fact -- is the WIMP. This stands for weakly interacting massive particle. A particle called the neutralino is one type of WIMP. Some scientists say that neutralinos were created in the Big Bang and that they contribute to most, if not all, of the dark matter.

Scientists at the University of Zurich have advanced this idea even further. They used a supercomputer to determine what these neutralinos might look like. They say that ghostly haloes of neutralinos as heavy as the Earth and as large as our Solar System were the first structures to form in the Universe. Our own Galaxy still contains quadrillions of these halos, with one halo expected to pass by Earth every few thousand years. Day to day, billions upon billions of random neutralinos rain down upon the Earth and through our bodies undetected.

There is a chance to detect these neutralinos with sensitive equipment. When two neutralinos collide, they make a "spark" of gamma-ray light.

It shouldn't surprise you that an exotic, invisible substance is filling the space all around you, between the walls and between your ears. Up until about 100 years ago, we didn't know about radio waves, X rays, neutrinos, cosmic rays and other forms of matter and energy. We might be only a few years away from directly detecting dark matter.

In 2007, NASA will launch a gamma-ray satellite called GLAST, which should be sensitive enough to detect neutralino sparks -- that is, if neutralinos exist. If not, GLAST and other new instruments will look for other types of dark matter, with names like axions, photinos and supersymmetry particles. Whatever the true nature of dark matter is, just reach out: You just grabbed a handful.

This week's question comes from Christopher Wanjek. Mr. Wanjek is a science writer supporting the Beyond Einstein initiative, a roadmap to understand the forces of nature beyond General Relativity and Quantum Mechanics through the study of the Universe from the Big Bang to black holes.