What is Dark Matter?
Updated: Jan 20, 2020
Fritz Zwicky was a brilliant, but abrasive, astronomer who worked at Caltech for 50 years of the 20th century. As a young and not yet established astronomer, he called out Robert Millikan, the head of his lab and Nobel laureate, by telling him he never had a single original idea. Over the years, he also referred to his colleagues as scatterbrains, thieves, horses asses, grey thinkers, and spherical bastards. Even students were not immune from Zwicky's abrasiveness as he took great pleasure in picking on them. During the 1931/32 academic year, in retaliation, a group of graduate students covertly got together and enrolled a bogus undergraduate student into Zwicky's class. Through the imaginary student, they achieved the impossible by earning a grade of A in the class. Unfortunately, no information is available about how Zwicky responded to this prank.
Fritz Zwicky was way ahead of most of his colleagues and his time. Only a year after the discovery of the neutron, he came up with the idea that some stars explode as supernovas and leave behind small dense cores composed primarily of neutrons. Zwicky proposed moving from passive astronomy to experimental astronomy. He suggested shooting the Moon, Venus, and Mars with nuclear weapons and observing the outcome with telescopes on Earth. He also suggested nudging Mars closer to the Sun to see if would become habitable. Decades later his suggestions became reality, at least in part. In 1967, graduate student Jocelyn Bell Burnell and her adviser Antony Hewish discovered the first neutron star. Then in 2005, NASA fired a projectile into a comet and observed it from the Deep Impact spacecraft and from telescopes on Earth. It is planned in 2022 that NASA will fly the Double Asteroid Redirection Test (DART) to change the motion of an asteroid. It’s a mission to test new technologies that could prevent an asteroid impact of the Earth.
Perhaps Zwicky’s most significant astronomical discovery was dark matter. In 1937, he analyzed the Coma cluster, a gigantic cluster of 1,000 galaxies, each galaxy containing between hundreds of millions to hundreds of billions of stars (see the image of the Coma cluster from the Lowell DiscoveryTelescope image in this article). By measuring the motions of the galaxies and applying the physics of Newton, he calculated the amount of mass in the galaxies. Let’s call it a dynamical mass. Next, he analyzed the brightness of the galaxies, and came up with a different measure of the mass. Let’s call in the luminous mass. The two should be the same. However, Zwicky found 500 times more dynamical mass than luminous mass. The vast majority of material in these galaxies was not emitting light. Zwicky referred to this mysterious and invisible material as dark matter. It would be decades before dark matter became accepted by astronomers. During these years, astronomers would find dark matter in huge quantities in nearly all galaxies, including our own Milky Way galaxy. It’s astounding to think that the vast majority of the observable Universe is made up of a mysterious material.
So, what is dark matter? There are three schools of thought on the nature of dark matter. The first school uses telescopes to look for objects that don’t emit much light, and so is hard to detect even with our biggest telescopes. These objects are known a massive astrophysical compact halo objects or MACHOs and include really faint stars or really faint remnants of stars, i.e. objects like black holes, neutron stars, brown dwarfs, white dwarfs, and red dwarfs. The most recent telescope observations are starting to tell us there just aren’t enough MACHOS to make up dark matter.
The second school tries to create dark matter in particle accelerators or detect it in vats of exotic liquid deep underground as it burrows through the Earth. Intensive world-wide efforts to create or detect dark matter in the form of scads of exotic theoretical particles with acronyms like WIMPS and SIMPS has so far failed to produce anything.
The third school says that dark matter doesn’t exist. What a cool solution, just get rid of the problem. The third school suggests that gravity doesn’t work the same way on the really large size scales of galaxies and clusters of galaxies as it does on the much smaller size scales of the Earth and our Solar System. In other words, if we better understood gravity, we would not need to invoke dark matter to explain work like Zwicky’s on the Coma cluster.
It’s fair to say that 85 years after Zwicky put forth the idea of dark matter, we still don’t have a clue what makes up dark matter or if it even exists. We need another Zwicky to help us forge ahead. Even then, it might take the rest of us grey thinkers decades or even much longer to catch up.