Dark matter is one of the central mysteries in astronomy. It’s estimated to comprise approximately 27% of the universe, but we can’t see it because it doesn’t emit light. In fact, scientists don’t have any direct evidence of the existence of dark matter, but they have circumstantial evidence—namely, the effects of its presence.
Something with mass—presumably, dark matter—speeds up the orbits of stars around their galactic centers. Astronomers have also observed gravitational lensing or the bending of light due to the gravity of an object they can’t see. This phenomenon allowed the Hubble telescope to capture a “ring” of what researchers believe is dark matter back in 2007.
Artist rendering of Earth surrounded by hairy dark matter. Credit: NASA/JPL-Caltech
Thus, scientists have for decades postulated the widespread existence of invisible matter or “missing mass.” A few years ago, NASA found more evidence in the discovery of 400,000 positrons thought to be the remains of dark matter collisions. Suffice it to say that astronomers are constantly on the lookout for direct or indirect evidence of dark matter.
To do this, scientists look to the past, or at light that has traveled millions of light years to become visible to their equipment. Recently, the Max Planck Institute for Extraterrestrial Physics and researchers from around the globe studied six galaxies that formed roughly 10 billion years ago. They thought they would find indications of the central role of dark matter, but they didn’t.
Ring of Dark Matter around galaxy cluster CL 0024+17. Credit: NASA/ESA
Instead, researchers found that these galaxies are “strongly dominated by normal matter.” Normal matter, also called baryonic matter, is the stuff we can see, such as gas, dust, and stars. They also found that the rotation velocities of these galaxies decrease farther away from their centers—but dark matter is thought to either speed up a galaxy’s rotational velocity as the radius increases or at least keep it constant.
Observations of another 200 galaxies also indicate higher levels of baryonic matter than anticipated, particularly in the form of gas, suggesting that dark matter perhaps plays a less crucial role in nascent galaxies than previously thought.
Collage of six cluster collisions with dark matter maps. Credit: NASA/ESA
Scientists believe that the gas in these early galaxies condensed into what they call “dark matter halos,” but that the dark matter within those halos didn’t condense until much later. This timeline would explain why dark matter doesn’t seem to exert much of an influence over the rotational velocities of early galaxies and why “present-day spirals, such as our Milky Way, require additional dark matter in various amounts,” according to study author Natascha Förster Schreiber.
Cosmos 3D Dark Matter Map. Credit: NASA/ESA/Richard Massey (California Institute of Technology)
While initially surprising, these findings make sense given what astronomers know about the formation of galaxies and the role they believe dark matter plays. Perhaps now that they know more about the relationship between timing and dark matter condensation, scientists will have better ideas about where and how to search for direct evidence of this elusive material.