What the Heck is Dark Matter?

September 9, 2015 | Sarah Tse

A ring of dark matter
Photo credit: The presence of dark matter, represented in blue, is inferred in this photograph. (NASA)

95 percent of the universe is concealed from us, but we are coming closer to understanding those substances.

Matter is everywhere. Although you can’t really feel or see it, the air is a thick soup full of atoms colliding with each other and emitting energy. In contrast, outside of the Earth’s atmosphere the universe seems largely empty.

But that ostensible emptiness isn’t supported by our data. The tiny amount of visible matter spread across the far-flung reaches of the galaxy can’t explain the behavior of stars and galaxies. Scientists realized that there had to be more to the universe than meets the eye: dark energy and dark matter, so called for their cryptic nature.

In Part I, we covered the ins and outs of dark energy, including recent breakthroughs in detecting it and its role in the fate of our universe. If dark energy makes up 70 percent of the universe, and normal matter is 5 percent, what about the remaining 20 percent?

That’s where dark matter comes in. Astronomers have found that galaxies have too little mass to spin as quickly as they do without scattering stars and dust like loose pebbles. Dark matter adds mass to those galaxies so they can exert a strong enough gravitational pull on their contents. While dark energy is responsible for pushing galaxies apart, dark matter holds them together.

The dominant theory on dark matter claims that it takes the form of weakly interacting massive particles, or WIMPs. But so far, detection experiments have failed to pick up the weak signals that would emit from a collision between a WIMP and a proton or neutron, called nuclear recoil. This made researchers turn their attention to electrons, which are far smaller and further from the atomic nucleus. If WIMPs preferentially bump into electrons, the resulting energy signal would be too feeble for most detectors to notice.

But now we have technology that is delicate enough to track down those beacons. The XENON100 experiment can differentiate between nuclear recoils and electron recoils. It uses a massive container of liquid xenon located deep underground to reduce interference by cosmic rays, which would otherwise conceal the faint energy signals emitted by electron recoils.

So far XENON100 has not detected any WIMP-electron recoils. But instead of being dismayed, scientists are thrilled with the detector’s sensitivity and capacity. These inconclusive results have narrowed down the field of possibilities for what WIMPs may look like, and will point us in the right direction for further investigation. Another experiment this fall will use an even more sensitive detector, XENON1T, that may finally unmask the true nature of dark matter particles.

Outside of lab-controlled studies, dark materials can also play a role in experiments on a much grander scale. The Dark Energy Survey provides data on how dark energy and dark matter behave in other galaxies both near and far. It uses an ultra-sensitive digital camera mounted high on a telescope, and takes photos of large portions of the sky. It is specially designed to probe for whispers of dark energy activity, such as supernovae, pressure waves of protons and electrons, changes in galaxy cluster counts, and light distortion.

The likelihood of finding evidence of dark matter is only increasing as we discover more dwarf galaxies—galaxies with fewer than 1000 stars that orbit larger ones such as our Milky Way. Because of their sizes, dwarf galaxies are thought to require large amounts of dark matter to exist stably, and in the past year, the Dark Energy Survey has already discovered a wealth of new ones. Future analysis of these new galaxies may yield deeper insights into how dark matter has helped to form galaxies.

At the same time as we analyze the role played by dark substances in both the past and future of the universe, we can hone our understanding of their actions as individual particles. The increasing delicacy of our detection technology will soon help us unravel the mysteries of these materials that so deeply define the fabric and flux of the universe.

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