Is Dark Matter Fuzzy?

The Cosmic Companion
Oct 3 · 4 min read

Dark matter is believed to be an underlying reason for the formation and development of galaxies in the early Universe. But was this dark matter cold, hot, or fuzzy?

Dark matter is makes up nearly 85 percent of the “matter” in the Universe, but it cannot be seen. Also, it does not give off heat, nor does it emit radio waves. The only way it can be detected is by the substantial effect gravity from dark matter has on normal matter — stars, galaxies, and everything else normally associated with the Universe around us. Astronomers can see the effects of this strange effect within galaxies, as well as between families of stars.

If dark matter did not exist, stars would fly away from galaxies, and galaxies themselves would not hold together in clusters. Dark matter also played a significant role in forming galaxies as these structures first formed in the early Universe.

Simulations of how galaxies would form in the early Universe if dark matter were (left to right) cold, hot, or fuzzy. Image credit: Universities of Princeton, Sussex, Cambridge

“Dark matter is a sort of cradle in which galaxies are born, and the cradle shapes also shape the galaxies. Different dark matter models predict different cradle shapes, particularly in the young Universe,” said Philip Mocz, Einstein Fellow at Princeton.

Which Flavor Dark Matter Would You Like?

Although the effects of dark matter are relatively simple to see, the underlying nature of this gravitational “something” remains a mystery. One school of thought looks at it as cold dark matter, moving slowly through space. This idea competes with another theory, hot dark matter. As might be expected, hot dark matter is thought to be lighter, and travel at faster speeds, than cold dark matter.

A contour map of dark matter, with lines representing the distribution of dark matter. Image credit: NAOJ/HSC Projec

A computer simulation from researchers at MIT, Princeton University, and Cambridge University has now examined, for the first time, how fuzzy dark matter may have helped shape ancient galaxies in the early Universe.

Fuzzy dark matter is a fairly new idea, centered on the idea of dark matter being composed of ultralight particles, so light that it would take one octillion (1,000,000,000,000,000,000,000,000,000) of these particles to reach the mass of a single electron. By comparison, the particle theorized to be at the center of cold dark matter is thought to have a mass equal to 10,000 electrons.

The extremely low mass of particles in the fuzzy dark matter theory allows the particles to act like a wave, like quantum matter. If this is confirmed, it would show the effects of quantum behavior acting over the distances between galaxies.

“The fuzzy dark matter theory, where dark matter behaves as a wave on galactic scales, now presents a credible alternative scenario: that dark matter is tiny, moves in waves behaving like quantum matter,” explains Dr. Anastasia Fialkov, who conducted the research while at the University of Sussex.

It’s like Looking for a Black Cat at Night by Following Debris

Although different dark matter models would not affect galaxies in the modern Universe, the nature of dark matter would have played a large part in the development of early galaxies.

The theory of cold dark matter suggests galaxies would have gathered together, forming halos. However, particle accelerators have, so far, been unable to create cold dark matter, which should be possible. The distribution of dark matter seen around very small galaxies is different than the cold dark matter theory predicts. These observations suggest other models may need to be explored in order to understand dark matter.

Both hot and fuzzy dark matter theories predict galaxies would form together in filaments. If ideas of fuzzy dark matter are correct, observations of galaxies in the early Universe will be seen as strings, like illuminated strings of a harp. Researchers also expect to see other observable traits of ancient galaxies which could be explained by the underlying properties of dark matter.

Simulations revealed how filaments of galaxies would form in the early Universe in the presence of (left to right) cold, hot, and fuzzy dark matter. Image credit: Universities of Princeton, Cambridge and Sussex

“Dark matter and dark energy are two things we measure in the universe that are making things happen, and we have no idea what the cause is.”
— Neil deGrasse Tyson

A new generation of telescopes currently being built could take a significant step forward in understanding ancient galaxies and the early Universe. These observatories, including the James Webb Space Telescope, should be able to determine if ancient galaxies formed in halos, straight filaments, or serrated ribbons, providing clues to the nature of dark matter.

A brief introduction to why we know that dark matter exists. Video credit: Science Museum (CC)

“The first galaxies in the early universe may illuminate what type of dark matter we have today. Either we see this filament pattern, and fuzzy dark matter is plausible, or we don’t, and we can rule that model out. We now have a blueprint for how to do this,” said Mark Vogelsberger, associate professor of physics at the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology (MIT).

The underlying nature of dark matter is one of the great mysteries currently being investigated by astronomers and astrophysicists today.

Analysis of the simulation was published in the Physical Review Letters.

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The Cosmic Companion

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James Maynard is the author of two books, and thousands of articles about space and science. E-mail:

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