4 min read

Dark Matter 101: Looking for the Missing Mass

Here’s the deal — here at NASA we share all kinds of amazing images of planets, stars, galaxies, astronauts, other humans, and such, but those photos can only capture part of what’s out there. Every image only shows ordinary matter (scientists sometimes call it baryonic matter), which is stuff made from protons, neutrons, and electrons. The problem astronomers have is that most of the matter in the universe is not ordinary matter – it’s a mysterious substance called dark matter.

A bright white spot at the center of the image is surrounded by a blue and red halo. The blue represents X-ray emission and appears as a thick “0” around the bright center. The red halo is optical and radio light, which fills the middle of the X-ray emission. Dotted around the image are additional white spots, which are galaxies and foreground stars.
This image shows X-ray data from Chandra (blue) of the Perseus galaxy cluster, as well as Hubble (pink) optical data and radio (red) emission from the Very Large Array.
X-ray: NASA/CXO/Fabian et al.; Radio: Gendron-Marsolais et al., NRAO/AUI/NSF; Optical: NASA, SDSS

What is dark matter? We don’t really know. That’s not to say we don’t know anything about it – we can see its effects on ordinary matter. We’ve been getting clues about what it is and what it is not for decades. However, it’s hard to pinpoint its exact nature when it doesn’t emit light our telescopes can see.

Misbehaving galaxies

The first hint that we might be missing something came in the 1930s when astronomers noticed that the visible matter in some clusters of galaxies wasn’t enough to hold the cluster together. The galaxies were moving so fast that they should have gone zinging out of the cluster before too long (astronomically speaking), leaving no cluster behind.

Two purple spiral galaxies rotate side by side. All the stars in the left-hand galaxy rotate at the same angular rate, like an old-fashioned vinyl record on a turntable. The stars in the right-hand galaxy rotate at different rates, depending on how far out from the center they are, with the fastest rotation toward the center. The result is a little like twirling pasta around a fork, where there’s a build-up of pasta/stars at the center.
This simulation shows two spiral galaxies with material orbiting based on two different mass models. The one on the left shows a galaxy where all of the visible stars rotate at the same angular rate, like an old-fashioned vinyl record on a turntable. The visible material in the galaxy on the right has different rotation rates, depending on how far out from the center they are, with the fastest rotation toward the center. We expected galaxies to exhibit rotation like one on the right, given mass estimates based on the luminous matter alone. However, observations have shown that galaxies rotate more like the galaxy on the left, pointing to a huge amount of matter that is unseen at any wavelength of light that we can detect.
ESO/L. Calçada

It turns out, there’s a similar problem with individual galaxies. In the 1960s and 70s, astronomers mapped out how fast the stars in a galaxy were moving relative to its center. The outer parts of every single spiral galaxy the scientists looked at were traveling so fast that they should have been flying apart.

Against the backdrop of a galaxy, a circular chart is drawn out showing one segment that’s 15% of the circle, which represents visible matter, and the rest, at 85%, representing dark matter.
This animated pie chart shows rounded values for the two known matter components of the universe: visible (15%) and dark (85%).

Something was missing – a lot of it! In order to explain how galaxies moved in clusters and stars moved in individual galaxies, they needed more matter than scientists could see. And not just a little more matter. A lot … a lot, a lot. Astronomers call this missing mass “dark matter” — “dark” because we don’t know what it is. There would need to be five times as much dark matter as ordinary matter to solve the problem.

Holding things together

Dark matter keeps galaxies and galaxy clusters from coming apart at the seams, which means dark matter experiences gravity the same way we do.

This animation shows light rays from a distant cluster of galaxies, on the right, as they are deflected above, below and around a source of dark matter, then continue on their way to the viewer. The viewer’s perspective is shown as flat image at the end of the light path, which turns toward the viewer showing the image of galaxies that are skewed in arcs around a central point.
Animation illustrating light from a cluster of galaxies being lensed by dark matter.
NASA's Goddard Space Flight Center/Conceptual Image Lab

In addition to holding things together, it distorts space like any other mass. Sometimes we see distant galaxies whose light has been bent around massive objects on its way to us. This makes the galaxies appear stretched out or contorted. These distortions provide another measurement of dark matter.

Undiscovered particles?

There have been a number of theories over the past several decades about what dark matter could be; for example, could dark matter be black holes and neutron stars – dead stars that aren’t shining anymore? However, most of the theories have been disproven. Currently, a leading class of candidates involves an as-yet-undiscovered type of elementary particle called WIMPs, or Weakly Interacting Massive Particles.

This animation shows a Weakly Interacting Massive Particle, or WIMP, not moving or reacting when an electron or proton pass near to it. Then another WIMP comes into view and they smash together, disappearing as a pair of gamma-rays are emitted.
WIMPs, or Weakly Interacting Massive Particles, represent a favored class of dark matter candidates. Some WIMPs may mutually annihilate when pairs of them interact, a process expected to produce gamma rays — the most energetic form of light.
NASA's Goddard Space Flight Center

Theorists have envisioned a range of WIMP types and what happens when they collide with each other. Two possibilities are that the WIMPS could mutually annihilate, or they could produce an intermediate, quickly decaying particle. In both cases, the collision would end with the production of gamma rays — the most energetic form of light — within the detection range of our Fermi Gamma-ray Space Telescope.

Tantalizing evidence close to home

In 2014, researchers took a look at Fermi data from near the center of our galaxy and subtracted out the gamma rays produced by known sources. There was a left-over gamma-ray signal, which could be consistent with some forms of dark matter.

This animation zooms into an image of the Milky Way, starting with visible light, which looks like puffy clouds of material in shades of light yellow with dark lanes between and stars, as tiny dots, densely packed throughout. At the center is overlaid a gamma-ray map as a purple circle with a bullseye with red at the very center surrounded by yellow, green and blue, representing the strength of the gamma-ray detections, with red being the greatest number of photons detected. When all the known sources are removed, a gamma-ray excess hints at the presence of dark matter.
This animation zooms into an image of the Milky Way, shown in visible light, and superimposes a gamma-ray map of the galactic center from NASA's Fermi. Raw data transitions to a view with all known sources removed, revealing a gamma-ray excess hinting at the presence of dark matter.
NASA's Goddard Space Flight Center, A. Mellinger (Central Michigan Univ.), and T. Linden (Univ. of Chicago)

While it was an exciting finding, the case is not yet closed because lots of things at the center of the galaxy make gamma rays. It’s going to take multiple sightings using other experiments and looking at other astronomical objects to know for sure if this excess is from dark matter.

Animation of the Fermi Gamma-Ray Telescope in space. The satellite features a large black box structure with white instruments underneath. Two long solar arrays extend from opposite sides, just under the black box.
NASA’s Fermi Gamma-ray Space Telescope, illustrated here, scans the entire sky every three hours as it orbits Earth.
NASA's Goddard Space Flight Center/Conceptual Image Lab

In the meantime, Fermi will continue the search.