Every galaxy we can see is believed to sit at the center of a huge cloud of invisible material called a dark matter halo.
These halos stretch far beyond the edges of the visible stars and gas in a galaxy.
Dark matter doesn’t shine or glow like stars do, but its gravity plays a key role in shaping the universe.
In fact, galaxies only form because dark matter pulls in gas, which then cools and forms stars. But scientists still wonder—are there some dark matter halos that never form any stars at all?
This idea of “completely dark” halos—dark matter clouds with no stars—has fascinated astrophysicists for years. Now, a new study may bring us closer to finding an answer.
Ethan Nadler, a computational astrophysicist at the University of California, San Diego, led the study, which was recently published in The Astrophysical Journal Letters.
Using computer models and theories about how galaxies form, Nadler explored what it takes for a dark matter halo to form stars.
Until now, scientists thought that halos needed to be fairly massive—between 100 million and 1 billion times the mass of our Sun—in order to cool down and create stars.
That’s because gas in these halos needs to cool before it can collapse into stars, and the cooling process depends on the type of hydrogen present. The earlier estimates were based on the cooling of atomic hydrogen, the more common form.
But Nadler’s study shows that a rarer form, called molecular hydrogen, can cool gas even in much smaller halos—as small as 10 million solar masses. This means that more tiny dark matter halos could possibly form stars than previously thought.
Still, there’s a lower limit. Below this new threshold, halos likely don’t form any stars at all. If we can detect such completely dark halos, it would be a major discovery. “Historically, our understanding of dark matter has been linked to its behavior in galaxies.
A detection of completely dark halos would open up a new window to study the universe,” said Nadler.
With new telescopes like the Rubin Observatory starting work soon and the James Webb Space Telescope already sending back detailed data, scientists may soon have the tools to test these predictions.
If completely dark halos are found, it could change how we understand the universe—and dark matter itself.
