Astronomers discovered the earliest known dark matter — and it's not behaving as predicted

Even though it’s impossible to see it, dark matter fills the universe. And now, it seems increasingly likely it always has. An international team from Japan used the Subaru Telescope at the controversial Mauna Kea Observatories complex to discover the earliest dark matter ever observed by tracing how it distorts measurements of the halos of millions of the oldest and most distant galaxies in the universe.

In a new paper published in Physical Review Letters, the team reports the earliest subtle traces of dark matter’s influence on galaxies in the young universe. They made the discovery after observing 1.5 million incredibly distant galaxies and their dark matter halos, peering back as far as 12 billion years.

What’s New — For the first time, these cosmologists show it is possible to use the cosmic microwave background itself — the radiation residue of the Big Bang — to measure halos of dark matter around extremely distant galaxies. As the mass of closer, more recent galaxies and their attendant dark matter bend the microwave background, astronomers can pick up on subtle fluctuations in the radiation to indirectly observe dark matter.

Andrés Plazas Malagón, one of the study team members and a researcher at Princeton University and the Vera Rubin Observatory in northern Chile, tells Inverse that looking at just one galaxy might not reveal much distortion at all. So the team combined observations of 1.5 million galaxies and the ring of dark matter surrounding each one to find a clearer signal.

“You don’t get information for every single halo,” he says. “but we expect that all these halos are very similar.”

“No one realized we could do this.”

The study diverges from other galaxy lensing investigations because it uses data from the cosmic microwave background rather than observing how light and radiation from more distant galaxies are influenced by nearer galaxies. Ultimately, the team observed dark matter from the first 1.7 billion years of the universe's existence. The discovery tells us that the earliest galaxies had halos of dark matter even as they first formed.

One of the study’s great advantages is the huge sample size — observing many galaxies makes very small but significant fluctuations in radiation easier to detect — but also, the data had already been collected. It’s possible other astronomers could use other existing large wide-survey datasets to discover more early dark matter — even these researchers were shocked they could pull this investigation off.

“Look at dark matter around distant galaxies?” Masami Ouchi, a professor at the University of Tokyo and study co-author, asks in an accompanying statement.

“It was a crazy idea. No one realized we could do this.”

Why it matters — Dark matter cannot be directly observed and its role in the universe is a mystery. Yet the substance is believed to make up a quarter of all existence. To better understand it, astronomers compare observations of our local universe with measurements of extremely distant and old objects.

Some observations tell you about the early universe, and some tell you about the current universe, and, according to Plazas Malagón, “to get from one to the other you use your mathematical model of the universe get your initial conditions, make a prediction, and that’s how you can make the comparison.”

Surprisingly, the dark matter observed in this study isn’t behaving the way the scientists had predicted based on what we know about the laws of physics, and in particular, the Standard Model, which offers a framework for characterizing all matter in the universe.

The best existing measurements of the cosmic microwave background were established by the ESA’s Planck mission a decade ago. And while it didn’t measure dark matter directly, the measurements set certain constraints on what physicists expect should be there. According to a popular cosmology theory called the standard Lambda-Cold Dark Matter model, dark matter should form locally dense clumps as a result of the random fluctuations in the cosmic microwave background and gravity. Instead, the team found early dark matter is less clumpy than expected.

If cosmologists’ expectations about the early universe don’t match astronomical observations, it’s possible other assumptions underpinning standard cosmology might break down the further you go back in time and space.

What’s next — For now, this analysis only involved a third of the galactic dataset collected by the Subaru Telescope. Astronomers could keep looking through the full dataset and other existing observations for traces of dark matter in extremely large samples of galaxies.

When the Vera Rubin Observatory in northern Chile where Plazas Malagón works starts observing the sky in 2023, it will be the largest digital camera in the world. The Observatory’s planned Legacy Survey of Space and Time, Plazas Malagón says, will be “like this survey but on steroids,” measuring the whole half of the sky visible from Chile. This means that it will capture billions and billions of galaxies, not the relatively mere millions captured by the Subaru Telescope or even the James Webb Space Telescope.

“The James Webb Space Telescope is amazing, but too small,” Plazas Malagón says.

“The James Webb Space Telescope is amazing, but too small.”

The team is also waiting for the end of the decade for a leap in their space-based observation capabilities — this is when the Nancy Grace Roman Space Telescope is expected to launch.

“For this analysis, we need hundreds of millions of galaxies,” Plazas Malagón says. “The Roman Space Telescope is going to give us a very large view.”

That the team’s findings in this study don’t match perfectly with what they expected mathematically doesn’t mean the theory of dark matter is problematic in and of itself or that the standard Lambda-CDM model isn’t still useful. While these observations are unexpected, Plazas Malagón notes that “it’s pointing us in the direction that there might be something into which we need to look further.”

“If you’re a good scientist, you want to break your model,” he says.