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\h        The Chandra X-ray Observatory orbits high above the Earth,

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\h        peering into the blackest reaches of space.

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\h        Exploring the most menacing and magnificent features of the

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\h        cosmos, this remarkable telescope is revealing what our eyes

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\h        can't, taking us beyond visible light.

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\h        Peering into the dark with its X-ray vision, NASA's Chandra X-ray Observatory is helping

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\h        to unravel one of astronomy's most perplexing enigmas.

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\h        Clusters of starlit galaxies and searing hot gas somehow stay together, even though the stars and gas

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\h        themselves don't have enough mass -- or gravity -- to explain their bond.

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\h        What cosmic ingredient is lurking in the darkness, undetectable by any of our telescopes,

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\h        holding these spinning groups together when they should be flung apart?

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\h        TANANBAUM: And, to hold the gas and the galaxies in place, you have to have this additional material.

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\h        You can compute how much there is -- it's about, again, close to 10 times more than we see in the gas and in the stars.

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\h        And this material, because it doesn't radiate x-rays, it doesn't radiate light, it doesn't radiate in the infrared –

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\h        we feel the force of its gravity, we know it's there but we can't see it. So we call it dark material, dark matter.

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\h        Light waves from normal matter can be detected by a variety of telescopes, such as optical,

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\h        X-ray, infrared, gamma-ray and more.

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\h        Dark matter, on the other hand, is visible only through its gravitational effects on the matter we can see around it.

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\h        In August 2006, Chandra and other telescopes working together found direct proof of dark

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\h        matter with a breakthrough discovery in a galaxy cluster known as the Bullet Cluster.

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\h        When the Bullet Cluster's galaxies merged, the clouds of gas slowed down due to friction,

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\h        but the galaxies themselves slipped through the collision.

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\h        TANANBAUM: The galaxies and the dark matter, they act more like individual particles,

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\h        and so they interact through the force of gravity.

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\h        But they don't behave like a gas or a fluid, and it tells us something about the interactions of the dark matter -- namely,

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\h        it doesn't interact with itself other than through the force of gravity.

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\h        And we have actually physically separated  -- -we haven't personally separated, but  in this Bullet Cluster –

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\h        the dark matter and gas have been separated -- and you actually can see the concentration of the dark matter

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\h        offset from the concentration of the gas. And it's a very visual demonstration that the dark matter

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\h        is real and it's different from the ordinary material, the baryons, the gas.

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\h        The mystery deepened once more in August 2007.

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\h        Chandra and optical telescopes revealed that in the Abell 520 galaxy cluster,

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\h        dark matter behaved in an opposite manner from the Bullet Cluster.

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\h        Instead of staying with the galaxies, the dark matter collected in the center of the cluster

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\h        while the bright galaxies collected outside the core.

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\h        Undaunted by this new finding, astronomers are not giving up.

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\h        VIKLINHIN: Also for studying dark matter, it's  important because most of the cluster

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\h        mass turns out to be in the form of dark matter. So, clusters are so big that the composition

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\h        of the clusters is exactly the same as that of the entire universe.

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\h        Using valuable tools like Chandra, the best scientific minds are working to reveal what