Space Matter: What You Should Know About Dark Matter

Space Matter is a weekly column that delves into space science and the mechanics of spaceflight. From the latest discoveries in the universe around us to the fits and starts of rocket test flights, you’ll find analysis, discussion, and an eternal optimism about space and launching ourselves into the cosmos.

You’ve heard a lot about dark matter. You probably know it’s not something we can see (hence the name). But if it’s not observable, how do we know it’s there? How did we discover it? What exactly is dark matter?

What Is Dark Matter?

Dark matter is basically just that: matter that does not interact with or give off any type of electromagnetic radiation: no light, no x-rays, no infrared waves, nothing that we can either see or detect through any type of traditional direct observation. But scientists theorize that the universe’s mass consists of roughly 85% dark matter—in other words, only 1/5 of the mass of the universe is the stuff we can see, hear, touch, and measure directly.

If We Can’t See It, How Do We Know It’s There?

This is the million-dollar question: How do we know dark matter exists, if we can’t detect it through any normal method? Well, just because dark matter doesn’t interact with the electromagnetic spectrum doesn’t mean that other forces don’t act upon it, or that it doesn’t influence the makeup of the cosmos. We know dark matter exists because it interacts with gravity.

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The ring seen in this image of galaxy cluster SDSS J1038+4849 is due to gravitational lensing. Photo: Courtesy of NASA/ESA/Hubble

Dark Matter’s Gravitational Effects

The galaxies in our universe are constantly in motion. That motion, and its characteristics, is determined by many factors, including the mass and energy within galaxies. The objects that are in galaxies—stars, planets, gas, plasma and more—are also in motion, rotating around the galaxy’s center. Think of our own solar system, with the Sun at its center, as a smaller example of this movement. In our (or any) solar system, the planets closest to the Sun move the fastest because the Sun acts upon them more strongly than it does the outer planets—the gravity of anything affects you more the closer you are to it. Yes, Mercury’s orbit takes less time than Jupiter’s, for example, because it’s closer to the Sun, but it also travels much faster in space. So, extending that logic, the stars and objects that are further away from a galaxy’s center (where most of its visible mass is clustered) should move much more slowly than anything that’s near it.

But that isn’t the case. Distance from center doesn’t seem to matter when it comes to the speed objects within galaxies travel. But rotation is determined by gravity, which is determined by mass. For galaxies to have the kind of mass that the actual galaxy rotation curve (the speed of stars versus their distance from the center) suggests (as opposed to the theoretical curve that we would predict, which would be consistent with how our own solar system works), there’s got to be a lot more mass than what we’re seeing—more than just stars, plasma/gas, and visible (or baryonic) matter. Otherwise, galaxies should be much, much brighter, filled with a lot more stars, gas, and plasma to account for this rotation speed.

That is, unless there’s missing mass that we just can’t see or detect, but that emits and is affected by gravity—dark matter.

How Can We Detect Dark Matter?

The most effective way for us to study dark matter is by looking at clusters of galaxies, because we have three different, independent ways of verifying their masses. Gravitational lensing is one of these methods. Let’s say we’re trying to look at a quasar (the region surrounding a supermassive black hole), and there’s a large object, such as a cluster of galaxies, in between us and the light source of the quasar. The cluster of galaxies will actually bend the light, as we see it, from the quasar; the amount it bends the light is determined by the theory of relativity and is dependent on the mass of the galactic cluster. In other words, we can determine the mass of a galactic cluster by how much it bends light from a distant light source. We can also measure the approximate mass of a galaxy or cluster by the light it gives off: The discrepancy between visible mass and actual mass is, basically, dark matter.

The Bullet Cluster is a cluster of galaxies that has been crucial to the study of dark matter—many scientists believe it proves the existence of the stuff, once and for all (though the majority of scientists are in agreement that dark matter exists, it’s still theoretical). The problem with observing dark matter is that it mingles with baryonic matter; it’s hard to tell the two apart. But what’s happening in the Bullet Cluster is two galaxies are (violently) merging. In 2006, a group of authors studying the Bullet Cluster discovered something shocking: by studying the gravitational lensing effects of the merging galaxies, they were able to determine that the visible matter has actually segregated from the dark matter. What’s more, they theorized that this wasn’t a singular phenomenon; the clustering of dark and baryonic matter was a normal byproduct of galactic collisions and mergers.

This galactic cluster is called the Bullet Cluster. While the concentration of visible (baryonic) matter is in red, the blue is dark matter. Photo: Courtesy of X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.

So What Does This All Mean?

Basically, it means that there’s still a lot to discover about the universe around us. We’re relatively certain that dark matter exists; the current standard cosmological model of our universe is called Lambda-CDM (LCDM), which incorporates the Big Bang, dark matter, dark energy, and a flat, expanding universe. It’s not the only model out there (and some scientists do argue that dark matter does not, in fact, exist), but it’s one that is most widely accepted.

Top photo: Courtesy of X-ray: NASA/CXC/SAO; Optical: Detlef Hartmann; Infrared: NASA/JPL-Caltech

Swapna Krishna is a freelance writer, editor, and giant space/sci-fi geek.