By Jade D. Pashoian

In the universe, more is unknown than known. Everything we experience is only a tiny fraction of reality. All that has been observed by scientists- what we’ve assumed is the actual universe, including all planets, stars, and galaxies- is known as baryonic matter. Baryonic matter makes up less than 5 percent of what’s out there. The rest of the universe is composed of mysterious, invisible substances known as dark matter and dark energy. Dark matter makes up 27% of the universe, and 68% is dark energy, which is an even bigger mystery.

What exactly are these scientific enigmas? They are quite hard to explain. For starters, astronomers predicted that the expansion of the universe would continuously slow down after the Big Bang, however, to their surprise, the universe’s expansion has been accelerating. This accelerated expansion of the universe is credited to a powerful force generated by quantum fluctuations in “empty space.” Empty space is not nothing, it can possess its own energy. Scientists declare dark energy to be the cause of the universe’s accelerating expansion. Dark energy was discovered in the 1990s and it is stronger than anything else known. Empty space has more energy than everything in the universe put together. Michael S. Turner, a University of Chicago cosmologist, ranks dark energy as “the most profound mystery in all of science.” Turner himself actually coined the term dark energy. Dark energy is a property of space, and it is invisible. Scientists have yet to discover a credible explanation for dark energy.

Dark energy and dark matter are not the same. Dark energy is causing the expansion of the universe to speed up and it pushes things apart. Dark energy has repulsive gravity. On the other hand, the gravity of dark matter is what holds the universe together. In the early 1930s, Fritz Zwicky, an astronomer at the California Institute of Technology, was studying how galaxies move within the Coma Cluster. He accidentally uncovered the gravitational effects of dark matter. The Coma cluster needed more than 400 times the mass Zwicky had calculated to hold itself together. Zwicky thus concluded that clusters must be held together by the gravity of unseen dark matter. Therefore, the Coma Cluster provided the first evidence for dark matter. The human eye cannot see that the Coma Cluster is filled with hot gas that emits x-rays. Most of the atoms that exist in a cluster of galaxies are in this hot gas. The term “dark” is relative because the hot gas that surrounds the galaxies can only been seen with x-rays. The x-ray gas accounts for most of the atoms in clusters. However, there is still a factor of 6 short for accounting for dark matter. So what information do we know about dark matter? Dark matter emits no light and cannot be seen directly. It is not stars, planets, black holes, or antimatter. It’s existence is certain because of it’s interaction with gravity. Dark matter is the reason galaxies can exist. If the matter scientists have observed was the only matter in the universe, galaxies would not have had enough matter to have ever formed, let alone hold themselves together. Zwicky’s idea was later confirmed by astronomer Vera Rubin in the 1970s. Vera Rubin showed that galaxies like the Milky Way and the Andromeda galaxy are filled with dark matter. She observed the velocity of stars orbiting around the center of the Andromeda galaxy. According to physics, stars at the edges of a spiral galaxy travel significantly slower than those near the galactic center. However, observations have shown that stars orbit at about the same speed regardless of their location. This points to the conclusion that the boundary stars are being effected by another mass– dark matter.

The existence of dark matter can be proven through gravitational lensing. This is an effect of Einstein’s theory of general relativity which states that just as a lens bends light, a large mass in outer space bends light traveling near it. It also states that the pull of gravity is actually caused by the bending of space. With gravitational lensing, the bending affects the light, distorting the image we see. Scientists can detect dark matter within a cluster of galaxies from the distortion it creates and the images seen of other distant galaxies. Dark matter cannot directly be seen, but the galaxies we can observe with telescopes form where there is a lot of dark matter. There are large amounts of dark matter within and between galaxies. If the bulk of dark matter is not made up of planets, stars, white dwarfs, green house gas, dust, or black holes, what is it? Dark matter is neither modifications of gravity nor ordinary matter. The preponderance of evidence reveals the existence of dark matter, but scientists have yet to prove what it actually is. Researchers believe that the bulk of dark matter is made of a new subatomic particle of nature, unlike protons, neutrons and electrons- which make up everything on earth. Many physicists consider that hypothetical subatomic particles, or weakly interacting massive particles (wimps), could make up the bulk of dark matter. Wimps do not interact with light, which is why they are a proposed possibility for the majority of dark matter. Axions might also be responsible for the bulk of dark matter. They have less mass than electrons, making them very small and light. Most physicists actually believe that 90% of the mass of every single galaxy in the universe is dark matter.

Though there have been several challenges in understanding dark matter and dark energy, their existence has been confirmed. Dark matter may reveal itself in the next 10 years, according to Gianfranco Bertone, a physicist at the University of Amsterdam in the Netherlands. Researchers are still searching for further evidence on both, but as of now, they remain the universe’s biggest mysteries.

Header image from The New Yorker.

References

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