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Today, we know dark matter as a mysterious, elusive presence in our universe, exerting a gravitational pull on our Milky Way and other galaxies. But in an unexpected twist, scientists are now learning dark matter was far less of a powerful presence in the early days of the universe than we once thought.

In a study to be published Thursday in Nature, as well as three accompanying papers in the Astrophysical Journal, an international team of researchers found that 10 billion years ago, the galaxy was dominated by baryonic or “normal” matter, which constitutes what makes up the gas, dust, stars, and planets in our universe today.

Today, dark matter, which does not emit, absorb, or reflect light, has a greater influence on our universe. It interacts with normal matter only through gravity, and it’s responsible for the rotation speeds in spiral galaxies.

“If you take a textbook case of a nearby spiral disk, there are large amounts of dark matter present that causes the rotational speeds to be high, even far from the galaxy center,” coauthor Stijn Wuyts of the University of Bath tells Inverse.

The team used telescopes to measure the rotation curves of six massive galaxies. Since these galaxies were so distant, it was like looking back in time 10 billion years ago, when stars were still forming.

This chart shows observations of six distant galaxies. The left column shows the distribution of the total surface brightness. The right column shows the velocity map. The galaxies show a rotation pattern, with blue areas moving towards the observer and red parts moving away.

But in early galaxies, researchers observed that the outer regions of the galaxies were spinning slower than central regions — meaning less dark matter than expected was present, and dark matter played a smaller role. These early galaxies were also more turbulent than spiral galaxies today. Two further studies of 240 star-forming galaxies supported these findings.

“From the peak epoch of galaxy formation 10 billion years ago to the present day, the reconstruction of this evolutionary path will be something we will be able to do more accurately,” Wuyts says.

Three to four billion years after the Big Bang, the gas in galaxies condensed into flat, rotating disks, while large dark matter halos surrounded them. Before, these halos were more spread out, but after billions of years, the dark matter has condensed and now has a larger effect on the rotation of many modern galaxies, like our Milky Way.

These findings have three possible implications. One is that because normal matter was more influential in the early universe, galaxies today have central regions that are rich in normal matter.

“It’s driven by the fact that galaxies in the young universe have so much gas in them, the gas that later in their lives will be used to form stars,” Wuyts says.

In addition, these findings imply that the winds driving out from the center of the galaxies may have altered the distribution of dark matter.

There’s also a speculative third possibility that the current hypothesis of dark matter may be incorrect. Perhaps the dark matter particle is lighter or not a particle at all, or perhaps it’s similar to black holes.

“We are learning that the situation is more complicated than people had assumed as far as dark matter is concerned, so we need to consider why that is,” lead author Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Germany tells Inverse. “If this might lead to a recognition that the dark matter paradigm needs to be rethought, that would be pretty sensational, but we need to first be conservative.”

The team saw the first hints of dropping rotation curves eight years ago, showing that dark matter was not as influential in the early universe. It continued to gather more evidence using data from other galaxies. In the next phase of their research, scientists plan to study galaxies with lower masses to help them better understand our universe’s origins.

“What I would predict is people will be very suspicious,” Genzel says. “That’s the nature of science. This is not what we would’ve expected. Other people will have to check it. In the meantime, the theorists hopefully will start thinking about the various options we’ve discussed and give us ideas on what we’ll do next.”

Abstract: In the cold dark matter cosmology, the baryonic components of galaxies - stars and gas - are thought to be mixed with and embedded in non-baryonic and non-relativistic dark matter, which dominates the total mass of the galaxy and its dark matter halo. In the local Universe, the mass of dark matter within a galactic disk increases with disk radius, becoming appreciable and then dominant in the outer, baryonic regions of the disks of star-forming galaxies. This results in rotation velocities of the visible matter within the disk that are constant or increasing with disk radius – a hallmark of the dark matter model. Comparison between the dynamical mass and the sum of stellar and cold-gas mass at the peak epoch of galaxy formation ten billion years ago, inferred from ancillary data, suggest high baryon factions in the inner, star-forming regions of the disks. Although this implied baryon fraction may be larger than in the local Universe, the systematic uncertainties (stellar initial mass function, calibration of gas masses) render such comparisons inconclusive in terms of the mass of dark matter. Here we report rotation curves for the outer disks of six massive star-forming galaxies, and find that the rotation velocities are not constant, but decrease with radius. We propose that this trend arises because of a combination of two main factors: first, a large fraction of the massive, high-redshift galaxy population was strongly baryon dominated, with dark matter playing a smaller part than in the local Universe; and second, the large velocity dispersion in high-redshift disks introduces a substantial pressure term that leads to a decrease in rotation velocity with increasing radius. The effect of both factors appears to increase with redshift. Qualitatively, the observations suggest that baryons in the early Universe efficiently condensed at the centres of dark matter halos when gas fractions were high, and dark matter was less concentrated.

Photos via Max Planck Institute for Extraterrestrial Physics, Flickr / European Southern Observatory

Each week we uncover the most interesting and informative articles from around the world, here are 10 of the coolest stories in science this week.

Minuscule extremophiles

**When tardigrades dry out, they retract their legs and heads, transitioning into a form called a "tun." This image shows a scanning electron micrograph of six tardigrades in their tun state.**
**Credit: T.C. Boothby**

Microscopic, blobby-bodied tardigrades — also known as "water bears" — are famed for their ability to survive in extreme conditions, even appearing to come back from the dead.

Now, a new study reveals that special proteins coded into tardigrade DNA may be the secret to the creatures' resuscitation superpowers. [Read more about the superpowers.]

Setting high records

The U.S. Air Force's robotic X-37B space plane, known as the Orbital Test Vehicle, is shown here in a 2009 photo at Vandenberg Air Force Base in California. An X-37B spacecraft is currently flying the fourth classified mission for the U.S. Air Force.
**Credit: U.S. Air Force**

The U.S. Air Force's X-37B space plane is just eight days away from setting a record on its current clandestine mission.

If the robotic vehicle stays aloft until March 25, it will break the X-37B mission-duration mark of 674 days, which was established back in October 2014. [Read more about the near record.]

Cheesy prints

**3D-printed cheese can hold its own against regular processed cheese, pictured here.**
**Credit: Binh Thanh Bui/Shutterstock**

Any way you slice it, cheese is considered by many to be a favorite food, whether cut into cubes as a snack, grated over pasta, layered in a sandwich or melted as a topping for pizza.

In a recent study, scientists 3D-printed cheese and conducted a series of tests evaluating its texture, resilience and "meltability," to see how this cheese from the future would stack up — on a structural level — against regular processed cheese. [Read more about printable cheese.]

Doomsday films

**A mushroom cloud forms after a nuclear weapons test dubbed Operation Hardtack-1 - Nutmeg in May 1958 on Bikini Atoll in the Pacific Ocean.**
**Credit: LLNL**

After decades spent slowly disintegrating in high-security vaults, thousands of historic films of U.S. nuclear weapons tests have been salvaged, including some that have been newly declassified. The incredible footage shows enormous mushroom clouds ballooning over the horizon in what could be a doomsday flick.

About 6,500 of the films have been located so far, and now, an initial collection of 64 videos, all showing tests conducted by the LLNL, have been made available online. [Read more about the declassified footage.]

First time users

**By monitoring subjects as they undergo adolescence, researchers could gain new insights into how marijuana affects developing brains.**
**Credit: Sean Gallup/Getty Images**

The chemical tetrahydrocannabinol (THC) in marijuana is known to trigger responses in brain regions related to thinking, perception, coordination and memory, and to have a lasting impact on users when taken frequently over time.

"We don't really know what happens to a first-time user — we don't know if one use will change the brain and make you more vulnerable to taking other drugs, for example," Weiss told Live Science. [Read more about pot brain.]

Hungry, hungry arachnids

A jumping spider, <em>Phidippus mystaceus</em>, feeding on a type of fly called a nematoceran. — **A jumping spider, *Phidippus mystaceus*, feeding on a type of fly called a nematoceran.**
**Credit: David E. Hill/Peckham Society, Simpsonville, South Carolina**

Each year, about 27 million tons of spiders consume somewhere between 440 million and 880 million tons of insects, new research finds.

Spiders are a very successful group of arthropods. They're found everywhere from deserts to grasslands to forests to Arctic tundra. More than 45,000 individual species have been identified so far, Nyffeler and Birkhofer wrote. Scientists estimate that there are around 131 spiders per every square meter of land on the globe, and in some places up to 1,000 individuals in that area (about the size of a single mattress). [Read more about their voracious appetites.]

Health and climate change

Climate change is poised to affect the health of Americans in every part of the country, a new report says.

But few Americans are aware of the impact climate change has on health, according to the report. Only about a third of Americans can name a specific way climate change affects people's health, according to the report. [Read more about climate change's affects.]

Life without the EPA

**A coal-burning power plant near Bismarck, North Dakota, on July 30, 2013.**
**Credit: Andrew Burton/Getty Images**

Acid rain devouring New England forests. Homes built on toxic sites. Unswimmable rivers and cities cloaked in smog. The United States looked very different before the Environmental Protection Agency (EPA) arrived, but a recent Congressional bill calls for the agency's abolition.

So Americans may wonder: What would life be like without the EPA? [Read more about the EPA's importance.]

Ancient plants

**X-ray tomographic picture (false colors) of fossil thread-like red algae.**
**Credit: Stefan Bengtson**

Scientists have discovered what may be the world’s oldest plant-like fossils, found in sedimentary rocks in central India. The preserved specimens are estimated to be 1.6 billion years old, and contain structures like those found in red algae.

The two types of fossils that researchers recently identified resembled red algae — one specimen was composed of filaments and another was made of more robust structures. The ancient specimens are 400 million years older than previous fossil algae discoveries, and hint that multicellular life evolved on Earth far earlier than was once thought. [Read more about the oldest fossils.]

Round tomb

**The tomb's entranceway is located on the south wall of the tomb. It was blocked off with bricks 1,000 years ago. Images of two servants can be seen flanking the entrance.**
**Credit: Courtesy of Chinese Cultural Relics**

A 1,000-year-old circular tomb, whose walls are decorated with colorful murals, has been discovered in Datong City, in northern China.

The murals on the walls show servants, cranes and numerous articles of clothing that hang on several stands, their colors still vibrant despite the passage of a millennia. [Read more about the vibrant murals.]

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