Avi LoebThe celebrated sculptor Greg Wyatt Working on the sculpture titled: “Galileo Galilei and the Four Moons of Jupiter.” (Image credit: Greg Wyatt)

The structures in our Universe were sculpted in invisible matter. We call it dark matter because it does not interact with light. This is surprising to us because we live on the surface of a spinning rock — the Earth, next to a luminous nuclear furnace — the Sun. The surface temperature of the Sun, 5800 degrees Kelvin, floods the dayside of the Earth with visible light. Detecting this natural light with our eyes as it bounces off objects in our immediate environment is helpful for avoiding predators and searching for food.

We live under a lamppost. But the Universe farther away is mostly dark, composed primarily of matter and energy that we cannot see.

Press enter or click to view image in full size(Image credit: Greg Wyatt)

The cosmos does not have us in mind. Despite our wishful thinking, we are not at its center, as Galileo Galilei realized by observing four moons orbiting Jupiter rather than Earth. Moreover, the Sun formed only in the latest third of cosmic history and there are 10^{21} Suns in the observable Universe. To imagine that we are important or alone is sheer arrogance.

Press enter or click to view image in full size(Image credit: Greg Wyatt)

Given its gravitational influence on visible matter, we can tell that 84% of matter is made of a substance that was never detected directly. The situation is similar to knowing about the existence of an unseen playwright from the motion of the visible actors on stage.

The first unambiguous evidence for dark matter was reported by Fritz Zwicki who nearly a century ago observed the dynamics of clusters of galaxies (as reviewed here). By now, it is evident that galaxies would not exist in the first place if all matter coupled to light, because the cosmic microwave background erased the seed inhomogeneities for visible matter on the scales of galaxies. The Milky-Way galaxy, inside of which we reside, would have never existed in the absence of dark matter. The Universe was sculpted in invisible matter, without which life would have not been possible. We owe dark matter existential gratitude.

Greg Wyatt Working on the sculpture titled: “Galileo Galilei and the Four Moons of Jupiter.” (Image credit: Greg Wyatt)

The nature of dark matter is unknown. Proposed candidates include weakly-interacting massive particles, axions, sterile neutrinos, primordial black holes, ultralight scalar fields, and strongly self-interacting particles. All viable models are constrained to reproduce the observed dark matter abundance, behave approximately as a cold (pressure-less) component during structure formation and remain stable over cosmological timescales. Dark matter particles must also have limited interactions with ordinary matter, or else we would have detected them in laboratory experiments.

Press enter or click to view image in full size(Image credit: Greg Wyatt)

Recently, a rare population of small galaxies was found to be deficient of dark matter. The first widely discussed example was NGC 1052 DF2, whose low internal velocity dispersion was found to be consistent with the gravity produced by its stars alone. NGC 1052 DF4 was later identified as a similar system in the same environment and more recently, FCC 224 in the outskirts of the Fornax Cluster was found to share several properties with DF2 and DF4. Another galaxy, NGC 1052 DF9, was reported to have weak gravity — close to the value expected from its stellar mass and well below the value predicted for a conventional dark matter halo. These galaxies are generally diffuse, have low internal velocity dispersions and contain old, largely quiescent stellar populations. Their common quality is that the dynamical mass inferred within the observed region is comparable to the mass present in stars and gas.

Press enter or click to view image in full size(Image credit: Greg Wyatt)

Various mechanisms were proposed to account for the dark matter deficiency in these rare galaxies. One possibility involves strong tidal stripping, wherein a dwarf galaxy may begin with an ordinary dark matter halo but lose much of its extended and weakly bound dark matter during repeated close passages around a massive host galaxy. Since the stellar component is usually more compact than its dark matter halo, it can remain bound after a large fraction of the halo has been removed. A second possibility is tidal formation, whereby stars and gas from massive galaxies during a major interaction can be drawn into tidal tails, where they clump into dwarf galaxies with little dark matter bound to them from the debris. A third possibility involves a high-speed collision of two galaxies in which the gas shocks and slows down, while the dark matter passes through freely. New dwarf galaxies may then form out of the compressed gas, with only a small fraction of the original dark matter.

The foundation for the bronze sculpture by Greg Wyatt, titled: “Galileo Galilei and the Four Moons of Jupiter.” (Image credit: Greg Wyatt)

A new paper posted here, that I co-authored with the brilliant PhD student from Vanderbilt University, Oem Trivedi, examines what dark-matter deficient galaxies can tell us about the nature of dark matter. A viable theory must allow the separation of ordinary matter from dark matter.

The new paper sets constraints on the properties of dark matter, including bounds on the abundance and cooling time of dissipative dark matter, and on the integrated escape rate of ultralight fuzzy dark matter from shallow gravitational potential wells. Dark matter deficient galaxies complement cosmological and laboratory probes by limiting late time dark matter interactions, dissipation and stability.

Science is work in progress. A month after establishing the UAP Science Advisory Council (https://uapsac.com/), I was urged by social media influencers to deliver results immediately. These commentators fail to recognize that scientific research takes time. In the case of the anomalous phenomenon of dark matter, we have been collecting data for nearly a century with an investment of billions of dollars in laboratory direct-detection experiments, but we still do not know what dark matter is. The only way to accelerate progress in our knowledge is by doing the hard work of data collection and analysis. Not through the easy work of commenting about it on social media.

Press enter or click to view image in full size(Image credit: Greg Wyatt)

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In this essay, I featured five amazing watercolors from a series created by the celebrated artist, Greg Wyatt. They include insights from Henry Wadsworth Longfellow and Arthur Eddington. This is the 26th in a sequence of essays, where Greg and I collaborate on the interface between art and science. The first essay in this series, titled “Music of the Cosmic Spheres,” appeared here; the second essay, titled: “Cosmic Waterfalls in Spacetime Cliffs,” appeared here; the third titled “Missing Elements in the Cosmic Jigsaw Puzzle,” appeared here; the fourth essay, titled: “Why Do We Exist?”, appeared here, and the fifth titled “Inspiration from the Stars”, appeared here, the sixth titled: “We Might Understand How the Cosmos Works Before We Understand How Life Works”, appeared here, the seventh titled: “Will the Human Survive for Billions of Years”, appeared here, the eighth titled: “The Butterfly Effect of Intelligence in the Cosmos”, appeared here, the ninth titled: “Benefits of Extraterrestrial Intelligence over AI”, appeared here, the tenth titled: “Übermenschen on Exoplanets” appeared here, the 11th titled: “If You Had an Infinite Research Budget, How Would You Allocate It?” appeared here, the 12th titled: “Are Human-Made Objects Orbiting Earth?” appeared here, the 13th titled: “Lets Send AI Astronauts, Not Humans, to the Moon”, appeared here, the 14th titled: “Our Highest Priority Should be National Innovation Centers to Complement AI Data Centers” appeared here, the 15th titled “The Cosmic Shells That Seeded Life” appeared here, the 16th titled “Chasing Alien Mysteries in the Sky” appeared here, the 18th titled: “The Smartest Investments in Our Future Are in Space” appeared here, the 19th titled: “The Best and Worst Are Yet to Come”, appeared here, the 20th titled “Would Alien Visitors Possess Intelligence Based on Silicon Chips or Synthetic Biology?” appeared here, the 21st titled “Cosmic Discoveries by National Security Sensors” appeared here, the 22nd titled “On Mysterious Orbs and Fireballs” appeared here, the 23rd titled “Neutrinos Are Worth a Thousand Words” appeared here, the 24th titled: “What is the Real Estate Value of the Neighboring Habitable Planet GJ 3378 b?” appeared here, and the 25th titled “A New Galileo Observatory in Celebration of Carl Sagan”, appeared here.

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ABOUT THE AUTHOR

Press enter or click to view image in full size(Image Credit: Lotem Loeb, May 22, 2026)

Avi Loeb is chair of the UAP Science Advisory Council to the White House, Pentagon, FBI and intelligence agencies, director of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, former director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.

Professional website:

https://lweb.cfa.harvard.edu/~loeb/

Social media:

https://avi-loeb.medium.com/
https://www.youtube.com/@ProfessorAviLoeb

https://open.spotify.com/show/1zhndXkvSY2b8FdjspFpCd
https://x.com/ProfAviLoeb