Looking throughout our universe, we see many fascinating objects and diverse phenomena. Observing over a range of wavelengths and distances, astronomers around the world have detected various planets, some possibly supporting life, as well as twinkling stars, evolving nebulae and clouds of gas and dust, exploding novae, spiraling and rotating galaxies, some colliding with each other, in addition to quasars, blazars, pulsars, comets, and other exotic things. But scientists argue that all of these light-emitting objects together only amount to an extremely tiny fraction in the universe!
According to the latest measurements from the European Space Agency’s Planck telescope earlier this year, they account for less than 5% of the universe’s matter and energy. Dark matter accounts for nearly six times as much, while the equally mysterious dark energy takes up the rest. If dark matter cannot be seen with any instrument or telescope, if it does not interact directly with normal matter or absorb energy from it, then why are astrophysicists so sure that it is out there in such abundant quantities?
The story of scientists’ discovery and exploration began at least eighty years ago. It was another tumultuous and exciting period for physicists, who were studying implications of Albert Einstein’s relatively new relativity theory and who engaged in heated debates about the nature of quantum mechanics, including the contentious issue of light behaving like both waves and massless particles (photons). In the 1930s, Swiss astrophysicist Fritz Zwicky studied the motions of hundreds of observed galaxies in the Coma cluster, over 300 million light-years away. It turns out that the galaxies appear to move too rapidly given their visible mass, but somehow the galaxies do not fly apart. Zwicky therefore inferred that some “dark matter” must be there to gravitationally hold the galaxies together.
Think of how much faster Mercury orbits our solar system’s sun compared to Pluto, which takes 248 Earth-years to complete an orbit. Based on the speed and distance of a planet from the sun and using Newton’s and Einstein’s gravitational laws, one can accurately estimate the sun’s mass. Similarly, one can make such calculations for galaxies within clusters or for stars within galaxies and infer the enclosed mass. In the 1960s and 1970s, American astronomer Vera Rubin measured and analyzed the precise velocities of stars in spiral galaxies and came to a startling conclusion. Most stars at outer radii orbit the center at surprisingly large speeds, much faster than they should be based on the mass of the stars themselves, but the galaxies do not tear themselves apart or fling their stars hurtling away. It was as if the galaxies contain and are surrounded by much more unseen dark matter, which gravitationally hold the galaxies together.
But the plot thickens. Although Rubin’s interpretation gradually became popular, the situation became more complicated as other astrophysicists developed alternative explanations. What if there are not large quantities of “exotic” dark matter? Israeli physicist Mordehai Milgrom argued that scientists’ understanding of gravity might be wrong at these great distances. He proposed a modification to Newtonian dynamics that could explain the rapid motions of stars in galaxy outskirts without invoking dark matter. Other “modified gravity” models followed and achieved additional successes.
One time when I was a struggling graduate student in the early 2000s, studying the distributions of hundreds of thousands of galaxies with dark matter models and contemplating these issues, I had a sudden sense of crisis. Feeling panicked, I ran into my advisor’s office and asked him: “if it turns out that dark matter does not exist, will all our years of work be wasted?” He reassured me and replied that our model’s framework would still be useful if this were the case, but in the meantime additional evidence for dark matter continued to accumulate.
In particular, astrophysicists found that the light from distant galaxies becomes distorted and magnified by nearer objects such as massive galaxy clusters, which act as gravitational lenses, like really big telescopes. Scientists use the effect to infer the masses of the lenses, which turn out to be much larger than the amount of visible matter. Furthermore, since galaxies form in massive clumps of dark matter, by analyzing the distributions of galaxies while surveying large swaths of the sky, astrophysicists can map out the dark matter, even though they cannot directly observe it. It is like how one might infer the orientation of a flock of birds from its shadow moving along the ground.
For example, following the groundbreaking observations of half a million galaxies with the Sloan Digital Sky Survey, new scientific collaborations with the Dark Energy Survey and the Kilo-Degree Survey, both utilizing observatories in Chile, observe numerous galaxies at optical and infrared wavelengths to reconstruct more detailed dark matter maps. At the same, scientists studying the cosmic background light with the Planck telescope and light from supernovae-bright fleeting explosions by dying stars, which provide precise distance measurements before they fade – also point to similar proportions of dark matter relative to luminous matter.
But what is the nature of these mysterious dark matter particles? Though much more abundant than luminous matter, dark matter continues to elude scientists. “Weakly interacting massive particles,” with the unfortunate acronym, WIMPs, constitute the most popular dark matter candidates, though this remains a subject of intense debate among the astrophysical community. Moreover, particle physicists seek to detect WIMPs with the recently restarted Large Hadron Collider, but these experiments would only amount to a first step toward narrowing down the likely candidates for dark matter particles. Based on many observations of light in the universe, scientists now know that dark matter certainly exists, but it may take more decades of research to pin down these elusive particles.
1 – Cosmological parameters from the Planck collaboration. 2015, Astronomy & Astrophysics, submitted.
2 – “Hubble Zooms in on a Magnified Galaxy.”
3 – Wide Field Lensing Mass Maps from the Dark Energy Survey. Vikram V., et al., 2015, Physics Review D, submitted.
4 – Science results from Sloan Digital Sky Survey.
Ramin Skibba is an astrophysicist and science writer based in San Diego, California. He hails from Colorado, and he studied galaxies, dark matter, and the expansion of the universe in his Ph.D. in Pittsburgh, Pennsylvania. He has worked as a research scientist and lecturer in Heidelberg (Germany) and Arizona, before coming to California. He is also passionate about science policy issues and outreach programs to encourage interest in science among young people, and he plans to pursue science communication full-time later this year.