Is Dark Matter Real?

roots of dark matter around Earth
Earth surrounded by filaments of dark matter called "hairs". These are the result of a theoretical study that suggests streams of dark matter travel the galaxy and may pass through our planet. NASA/JPL

Dark matter is very mysterious stuff in the universe. It turns out to be an incredibly important part of the cosmos, but It can't be seen or felt. It can be detected by telescopes or other instruments. Dark matter has been around since the beginning of the universe and played an important role in the evolution of stars and galaxies.

Oddly enough, however, it wasn't really noticed by astronomers until they began studying the motions of galaxies. Dr. Vera Rubin and her team observing galaxy rotation rates in the mid-20th century measured those motions and noticed that galaxies weren't spinning at the expected rates. Too much mass was needed to explain the rotation rates they were measuring. This isn't logical, given the amount of visible mass and gas that can be detected in galaxies. There HAD to be something else there.

The most likely explanation, it seemed, was that there must be mass there that we can't see. It turned out that it would have to be a lot of mass — about five times as much mass already seen in a galaxy. In other words, about 80% of the "stuff" in these galaxies was dark. Unseen. 

Birth of Dark Matter

Since this new matter clearly did not interact electromagnetically (i.e. with light), it was dubbed dark matter. As astronomers began to study the interaction of galaxies, they also noticed that galaxies in clusters in particular were behaving as if there was a lot more mass in the cluster. 

Techniques were used to measure the gravitational lensing — the bending of light from distant galaxies around a massive object in between us and the galaxy in question — and found a significant amount of mass in these galaxy clusters. It just wasn't being detected any other way.

Problems With Dark Matter Theories

There is certainly a mountain of observational data to support the existence of dark matter. But there are some merging galaxy cluster systems where the dark matter model can't seem to explain the anomalies.

Where does dark matter come from? That's a problem, too. Nobody is sure how or where it formed. It doesn't seem to fit nicely into our standard model of particle physics, and simply looking at objects like black holes and other objects don't fit some of the more convincing astronomical data. It had to be in the universe from the beginning, but how did it form? No one is quite sure...yet.

Our best guess so far is that astronomers are looking for some kind of cold dark matter, specifically a particle known as a weakly interacting massive particle (WIMP). But, they don't know how such a particle would be made in nature, only that it would need to have certain properties.

Detecting Dark Matter

Finding a way to detect dark matter is an uphill battle, partly because astronomers don't even really know what it is they're looking for. Based on the best models, scientists have come up with clever experiments to detect dark matter as it passes through Earth.

There have been some detections of something, but physicists are still analyzing just what happened. It's difficult to do this work since the particles, by definition, do not interact with light which is the primary way that we make measurements in physics.

Scientists also look for dark matter annihilations in nearby galaxies. Some theories of dark matter claim that WIMPs are self annihilating particles, meaning that when they encounter other dark matter particles they convert their entire masses into pure energy, specifically gamma rays.

However, it's not clear if this property is true of dark matter. It's very rare for self annihilating particles to exist in nature at all. Even if they do, the signal would be very weak. So far, gamma-ray experiments have been unsuccessful in detecting such signatures.

So Is Dark Matter Real?

There is a mountain of evidence that dark matter is actually a form of matter in the universe. But there is still a lot that scientists don't know. The best answer is that there appears to be something, call it dark matter or whatever, that is lurking out in there that we have yet to measure. The alternative is that something is seriously wrong with our theory of gravity. That, while possible, would itself have a difficult time explaining all of the phenomenon that we see in galaxy interactions. Only time will tell.