Did you know that a staggering 80% of the universe is made up of material that scientists can’t actually observe? It’s known as dark matter, something that doesn’t emit any light or energy. In that case, why have scientists been left to believe that it is the most dominant material in our universe?
If you can’t see it, how do you know it exists?
When studies of other galaxies were carried out back in 1950, it was indicated that the universe contained more matter than could possibly be seen by the naked eye. Since then, despite there being no solid evidence for its existence, support for dark matter has grown.
Scientists calculate the mass of large objects in space by studying how they move. Those examining spiral galaxies in the 1950’s were expecting to see material in the center moving faster than that in the outer edges. Instead, it was found that the stars in both locations traveled at the same velocity. This indicated that the galaxies contained more mass that could actually be seen.
Further studies of the gas within elliptical galaxies also indicated the need for more mass that was found in visible objects. Clusters of galaxies would actually fly apart if the only mass they contained were visible to our current conventional astronomical measurements.
Albert Einstein’s studies also showed that massive objects in the universe bend and distort light meaning that they can be used as lenses. Therefore, by studying how light is distorted by galaxy clusters, astronomers could create a map of dark matter in the universe.
What is dark matter made of?
There have been a few strong possibilities over recent years. Pieter van Dokkum, a researcher at Yale University, led a team that identified the galaxy Dragonfly 44, a place composed almost entirely of dark matter. In a statement he said, “motions of the stars tell you how much matter there is.”
The most familiar material of the universe, known as baryonic matter, is composed of protons, neutrons, and electrons. Dark matter can be made up of both baryonic manner or non-baryonic manner but to hold the elements of the universe together, dark matter must make up approximately 80%.
This missing matter may simply be more difficult to detect and made up of baryonic matter. Potential candidates for this include dim brown dwarfs, white dwarfs, neutrino stars or even supermassive black holes. However, these objects can be incredibly hard to spot and would have to play a much more dominant role than scientists have observed.
For that reason, most scientists believe that dark matter is composed of non-baryonic matter including WIMPS (weakly interacting massive particles) that have ten to a hundred times the mass of a proton but have weak interactions with normal matter, making them harder to detect. Neutralinos, massive hypothetical particles heavier and slower than neutrinos, are the number one candidate but are yet to be spotted.
Experiments to suggest its existence
Now that we’ve explained how dark matter is different from ordinary matter, we can take a look at how astronomers are working to detect such an unusual material.
The Alpha Magnetic Spectrometer (AMS) is a sensitive particle detector on the International Space Station and has been operating since 2011. So far, it has tracked more than 100 billion cosmic ray hits in its detectors. AMS lead scientist, Samuel Ting, a Nobel laureate with MIT said, “We have measured an excess of positrons [the antimatter counterpart to an electron], and this excess can come from dark matter. But at this moment, we still need more data to make sure it is from dark matter and not from some strange astrophysics sources,” Ting said. “That will require us to run a few more years.”
On Earth, underneath a mountain in Italy, the LNGS’s XENON1T hunts for signs of interaction when WIMPs collide with xenon atoms.
“A new phase in the race to detect dark matter with ultra-low background massive detectors on Earth has just began with XENON1T,” project spokesperson Elena Aprile, a professor at Columbia University, said in a statement. “We are proud to be at the forefront of the race with this amazing detector, the first of its kind.”
The Large Underground Xenon dark-matter experiment (LUX), seated in a gold mine in South Dakota, has also been hunting for signs of WIMP and xenon interactions but is still yet to reveal any of the mysterious matter.
Other instruments on the hunt include the European Space Agency’s Planck spacecraft, which has been working on a map of the universe since 2009.
Is dark matter the same as dark energy?
Of course, it’s important not to confuse dark matter with dark energy. Although dark matter makes up most of the matter of the universe, this is only about a quarter of the composition. Dark energy dominates the rest.
After the Big Bang, the universe began to expand outward. Scientists once believed that this would mean it would eventually run out of energy, slowing down as gravity began to pull the objects inside it together. However, studies of a distant supernovae revealed that the universe is actually expanding outward much faster than it ever did in the past. This is something only possible if the universe contained enough energy to overcome gravity. Thus, we have dark energy.
