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These images are a composite of separate exposures of the “little red dots” acquired by the James Webb Space Telescope using its primary near-infrared imager, which provides high-resolution imaging and spectroscopy for observing the early universe.

Image: NASA, ESA, CSA, STScI, Dale Kocevski (Colby College)

Kermit Pattison

Harvard Staff Writer

September 4, 2025 5 min read

Astrophysicists think mysterious ‘little red dots’ are generated from spinning dark matter, studying them may yield insights into evolution of universe

For more than two years, astronomers have been puzzled by a mysterious discovery from the ancient universe — hundreds of objects known as “little red dots” so far away the light had to travel billions of years to become visible to scientists.

First detected by the James Webb Space Telescope, these unusually compact vestiges from the cosmic dawn have sparked intense debate: Are they densely packed galaxies? Or do they contain massive black holes?

Now two Harvard astrophysicists have proposed a new theory: These distant objects are new galaxies being formed inside slowly spinning halos of dark matter — and studying them may yield important new insights into the formation of the universe.

“Telescopes are time machines,” said Fabio Pacucci, a Clay Fellow in the Harvard-Smithsonian Center for Astrophysics and first author of the new paper. “If you look at the moon, you see it as it was one second ago, and if you look at the sun, as it was eight minutes ago. If you look at these little red dots, it was billions of years ago.”

Dark matter halos are believed to play prominent roles in the birth of galaxies and the evolution of the universe. (Dark matter is a mysterious substance that remains invisible because it does not absorb, reflect or emit light, but it is believed to comprise the vast majority of matter in the universe.)

“If you look at the moon, you see it as it was one second ago, and if you look at the sun, as it was eight minutes ago. If you look at these little red dots, it was billions of years ago.”

Fabio Pacucci.

File photo by Aaron Ye

The halos have not been directly observed, and their existence is based on inferences from other observations, including the motions of stars and gas, and the bending of light.

“A dark matter halo is a cradle to form a galaxy,” said Pacucci. “The bigger the dark matter halo, the bigger the galaxy at the center.”

The enigmatic dots are the most startling discoveries from the most powerful telescope ever launched into space. Launched in 2021, the James Webb Space Telescope (JWST) was designed to study the “cosmic dawn” — the epoch when the first stars and black holes were born after the Big Bang 13.8 billion years ago.

Orbiting the sun about 1 million miles away from Earth, the JWST spotted hundreds of unusually red and compact sources dubbed the “little red dots” (LRDs).

Their distinctive color is caused by a combination of effects, including the presence of dust and the phenomenon of “redshift” (in which light shifts to the red end of the spectrum as it travels vast distances).

LRDs appeared about 600 million years after the Big Bang and then later vanished.

The dots are unusually compact and relatively bright, suggesting they either host enormous black holes (which shine brightly despite their name) or pack inconceivable numbers of stars into galaxies only one-fiftieth the size of our Milky Way.

“They are like cosmic fireworks,” said Pacucci. “They magically appear, and they are very visible for about 1 billion years. Then they just disappear.”

In a paper published recently in The Astrophysical Journal Letters, Pacucci and Abraham “Avi” Loeb, Frank B. Baird Jr. Professor of Science, propose the theory of galaxies being formed inside slowly spinning halos of dark matter to explain the abundance, compactness, and redshift distribution of LRDs.

In their model, the LRDs discovered thus far represent only the very slowest-spinning galaxies — the bottom 1 percent of the distribution.

In other words, the LRDs are not a fundamentally distinct population of galaxies, but just a small subset that exhibit unusual properties.

Some of their seemingly mysterious features may arise from observational bias: Current technologies only can detect compact, low-spin halos because they concentrate light in bright cores; meanwhile larger, more diffuse galaxies at higher redshifts remain invisible — despite being more common.

Loeb said, “If you assume the little red dots are typically in the first percentile of the spin distribution of dark matter halos, then you explain all their observational properties.”

Dale Kocevski, a leading researcher of LRDs and chair of the physics and astronomy department at Colby College, said the theory proposed by Pacucci and Loeb “makes a lot of sense.”

“This potentially adds to our fundamental understanding of these objects,” he said. “In addition, it provides a physical model that we can test going forward.”

Pacucci believes that the LRDs eventually will prove to be the signature discovery of the JWST. (Like many colleagues, he also suspects the little red dots do contain supermassive black holes, but that is not part of the new theory.)

He predicts they will generate new insights about the formation of galaxies and black holes during the cosmic dawn.

“We are now debating what is the nature of a fundamentally new kind of galaxy that we’ve never seen before,” Pacucci said. “This will fundamentally change how we view the early evolution of the universe.”