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Sky Sighting: Is That A Thread Of Dark Matter I Spy?

A tenuous thread of dark matter is seen connecting the galaxy clusters Abell 222 and 223.
Courtesy Jörg Dietrich/Universitäts-Sternwarte München
A tenuous thread of dark matter is seen connecting the galaxy clusters Abell 222 and 223.

When astronomers survey the universe, the landmarks are galaxies, those gigantic agglomerates of stars and interstellar gas spread across the immensity of space. A typical spiral galaxy, like our own Milky Way, boasts hundreds of billions of stars grouped along hundreds of thousands of light-years. That means that it takes a beam of light all that time to go from one extreme of the galaxy to the other, traveling, as light does in a vacuum, at 186,282 miles per second.

But galaxies are much more than what the eyes can see. Nested in their center are enormous black holes, some with masses equal to many millions of suns. Those behemoths gobble up matter around them, creating the bright "eyes" that we see in telescopic pictures such as this one. As matter falls into the black hole, it radiates energy, which telescopes of different types (from optical to radio and X-ray) detect.

And then there is the invisible stuff.

An invisible cloak of what is called "dark matter" surrounds most galaxies, a type of matter of yet unknown composition. We know it's not made of protons and electrons, like ordinary matter. The only way we detect the presence of dark matter is by its gravitational effect on normal matter: Dark matter does have mass and thus pulls on normal matter. By mapping the motions of stuff made of normal matter, we can infer the presence of dark matter.

Dark matter plays a key role in the structure of the cosmos. In fact, without it we wouldn't understand much about the formation of galaxies and their observed distribution in space.

It turns out that galaxies are distributed along invisible filaments and voids, as if there were some kind of ethereal blueprint across the volume of space resembling the froth in a bubble bath. After decades of astronomical observations and computer simulations, scientists agree that this blueprint is mainly made out of dark matter, forming a kind of cosmic web.

Going back to when the universe was very young, just thousands of years after the Big Bang, particles of dark matter started to cluster, as if falling into wells. These wells, in turn, were the result of much earlier cosmological processes during a phase called "inflation," characterized by a very fast expansion of the universe. During inflation, tiny lumps of energy existing in the matter at the time got amplified to large scales and, after some 50,000 years, started to attract the dark matter particles.

Theorists predict that since dark matter doesn't emit light or any other form of electromagnetic radiation, gravity can't make it collapse very efficiently, as it can with ordinary matter. If dark matter is made of some kind of unknown particle, it probably won't form stars. As a consequence, the best that it can do is coalesce here and there into huge blobs and get stretched here and there into threads, forming an invisible web across space. But can this invisible web be seen?

Recently, astronomer Jörg Dietrich from the University Observatory of Munich and colleagues have spotted a tenuous dark matter bridge linking two relatively close clusters of galaxies, Abell 222 and Abell 223, a kind of invisible umbilical cord connecting two very luminous sights. Their results were published in the journal Nature July 12.

In order to see what's invisible, the team used a remarkable effect that Einstein predicted would exist if his theory of gravitation as curvature of space were right: Light from a distant object will bend as it passes near a massive one, distorting the original image. This phenomenon, called "gravitational lensing," is one of the best ways to detect dark matter through its gravitational effects.

Sure enough, Dietrich and colleagues found distorted images of galaxies along a thread connecting the two clusters. Gas could only account for 9 percent of the matter along the filament. The rest is made of dark matter.

If we extrapolate these findings, quite possibly the whole of space is connected through an invisible web of dark matter. The galaxies and clusters of galaxies that we see through their emitted light or other form of radiation tend to collect at the nodes of the cosmic dark matter web. As the Greek Presocratic Heraclitus mused, "Nature loves to hide." In turn, scientists love to find its hideouts.

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Marcelo Gleiser is a contributor to the NPR blog 13.7: Cosmos & Culture. He is the Appleton Professor of Natural Philosophy and a professor of physics and astronomy at Dartmouth College.