By the Numbers: Dark matter

Key facts and figures about the unseen matter that remains one of cosmology’s greatest unsolved mysteries.

a cluster of galaxies shown against a dark blue background, a faint darker ring can be seen around the center of the image
This image taken by the Hubble Space Telescope depicts a massive ring of dark matter surrounding the center of the CL0024+17 galaxy cluster. The computationally-modeled and superimposed dark matter ring spans about five million light years and is hypothesized to exist based on observed distortions of the distant galaxies. (Image: NASA, ESA, M. J. Jee and H. Ford et al., Johns Hopkins University)

With a number of celebrations focused on the ethereal happening this time of year, from Día de los Muertos to Halloween, Oct. 31st is also a day for celebrating one of the most mysterious phenomena of the universe: Dark matter. 

Scientists and astronomy enthusiasts around the world will celebrate Dark Matter Day by reflecting on the ongoing efforts of the observational astronomers and theoretical physicists who are working to understand the unseen forces that shape the structure and nature of the universe. 

Penn Today delves into key facts and figures on what dark matter is, ongoing research that seeks to gain a better understanding this unseen force, and Penn’s role in this cutting-edge field of study.

    • 85%

      The amount of matter in the universe thought to be made of dark matter. Scientists refer to this type of matter as “dark” because it doesn’t interact with light or other parts of the electromagnetic spectrum, so it can’t be detected with telescopes. 

      Scientists first knew about dark matter because of observations of spiral galaxies, which found that stars that were farther away from the center of the galaxy travelled faster than expected. Since galaxies weren’t flying apart, astronomers invoked dark matter as an additional source of gravity that was holding stars on their regular orbits. 

      In Kepler’s laws of planetary motion, the velocity of an object in orbit decreases the farther away it is from the center, a phenomenon seen in the solar system, with Jupiter and Saturn taking longer to travel around the sun that Earth or Mars. But this wasn’t the case for spiral galaxies: The velocity barely decreased, even farther out from the center where there were hardly any stars. This led researchers to speculate that dark matter actually dominated the gravity at large distances. 

      Other astronomical observations, like how galaxies clustered together when the universe was forming, imprints on the cosmic microwave background, and the amount of gravitational lensing, or how much light “bends” between massive objects at long distances, also lend support for the existence of dark matter. 

    • 28,999

      Number of papers on the arXiv that mention “dark matter,” which represents 1.8% of the entire arXiv database. The arXiv is a preprint server for research papers from the natural sciences, computer science, statistics, engineering, and economics, where 1.6 million articles have been posted since 1992.

      The 2019 Nobel Prize in Physics was awarded to James Peebles for his theoretical work in physical cosmology, and Peebles’ research during the 1980’s provided support for the existence of dark matter. He was also one of the first researchers to publish the theory of cold dark matter, which hypothesizes that dark matter moves slowly when compared to the speed of light.  

    • 7.5 billion

      Number of years ago when objects in the universe began flying apart from one another at a faster rate. This shift towards accelerated expansion, discovered in the late 1990s, led scientists to hypothesize that an unseen form of energy could explain this phenomenon: dark energy. 

      To help explain how this acceleration continues to pull galaxies apart, countering the opposing force of gravity, scientists are also working on theories and running experiments to understand the nature of this second mysterious cosmological entity. So far, the source of most of the matter and energy in the universe remains largely unknown. 

    • 5

      The number of episodes of “Star Trek” that feature dark matter, one example being the “In Theory” episode of “Star Trek: The Next Generation,” where the Enterprise finds a nebula made of dark matter that causes distortions in space. 

      This episode originally aired in 1991, a time when dark matter theories were starting to become well-established in the scientific community and as major observational experiments, like the Cosmic Background Explorer, started to provide supporting evidence for the existence of dark matter. 

    • 30+

      Number of Penn students, post-docs, and faculty involved with dark matter and dark energy research, including researchers in the labs of Gary BernsteinMark DevlinJosh KleinBhuvnesh JainElliot LipelesChristopher MaugerMasao Sako, and Mark Trodden

      Recent work from Penn about dark matter and dark energy includes a new hypotheses on the phase of dark matter, evidence for the boundary of dark matter “halos,” and developing the largest contiguous maps showing the concentration of dark matter in the cosmos

      Researchers at Penn are also analyzing the 50 terabytes of data collected during the Dark Energy Survey, the largest galaxy survey to date, which completed six years of observations in January. Penn scientists will use this data to see if their theories about dark matter and dark energy align with the data collected during the survey. They hope to shed light on recent uncovered puzzles on the expansion of the universe and the distribution of matter. 

The Dark Matter Day website has additional information about dark matter and events celebrating the occasion.