The Unseen Universe - What is Dark Matter?

The cosmic web, the structure of universe. This delicate structure is only possible due to dark matter serving as a scaffolding. At this scale, even galaxies are miniscule. The individual lights are massive galaxy clusters.

The Mystery

Everything you can see and touch is made up of matter. Matter is a fundamental building block of the universe. But, somehow, 85% of it is missing. When we look at all the stars, planets, asteroids, and more in a galaxy, we can calculate its total mass. But mass is the source of gravity. And when we measure the gravitational effects of that same galaxy, we get an entirely different mass. The two methods of determining the mass of a galaxy are at odds with one another, and we can’t get them to agree. This is where dark matter comes in.

The Suspects

The simplest definition of dark matter is matter that we can’t see. It is what would account for the 85% of mass missing from our direct observations. There are two primary candidates for dark matter. First, there’s the Massive Compact Halo Object (MACHOs), which would simply be ordinary (at least relatively ordinary) matter found at the edges of galaxies, known as halos. There is little light from stars in the outer galactic halo, and these objects, despite being massive, would emit little to no light of their own, making them next to impossible to detect, outside of their gravitational interactions with luminous matter. Rogue planets, brown dwarfs, neutron stars, and black holes, are all potential MACHO candidates.

We can detect MACHOs by their gravity warping the light of a star they pass in front of, and the results are clear. There aren’t nearly enough MACHOs in the average galaxy to account for the missing 85% of matter. It is generally agreed that MACHOs are not the primary type of dark matter. We need something even more exotic for that.

The opposite of a MACHO is (obviously) a WIMP, or a Weakly Interacting Massive Particle. Yes. Physicists love their fun acronyms. Unlike a MACHO, a WIMP isn’t just invisible due to a lack of light, but rather because it doesn’t even interact with light. If you found a (non-black hole) MACHO, you could shine a light to see it or even reach out and touch it. But not a WIMP. Whether it’s a beam of light or your fingers, it will just pass straight through. It doesn’t matter what you do, the only way to ever tell a WIMP is there is by its gravitational interactions. When you hear about dark matter in astrophysics, the topic is almost certainly focused on WIMPs or some similar exotic particle.

A WIMP has mass, and therefore gravity, but it doesn’t interact with regular matter or energy in any other way. Trillions of WIMPs could be flying through your body, and you would never know. In fact, they could pass through a mile thick wall of lead easier than you walk through air. However, it is theoretically possible that two different WIMPs could interact with each other, potentially destroying both particles and creating a burst of gamma rays in the process. This is how scientists hope to detect WIMPs. To date, WIMPs remain entirely hypothetical, as no direct evidence of one has ever been detected.

The Evidence

Despite this, we do have evidence of non-interacting dark matter. In 2006, scientists studying the Bullet Cluster, a galaxy cluster formed by the collision of two other high-velocity clusters, noticed a discrepancy. A majority of visible matter of the clusters remained at the center of the point of impact, as their collisions slowed it down, but surrounding that matter was a large amount of gravity, measured by the way it distorted the light of background galaxies, that seemed to come from nowhere. As the dark matter didn’t interact with the regular matter, it was able to keep on moving at the original velocity of the two clusters, leaving the rest of the galaxies behind and becoming that invisible source of gravity.

The Bullet Cluster with normal matter shown in pink, and gravity revealed by gravitational lensing in blue. While we can't see the dark matter in the process of speeding away from collision of clusters in either direction, we can map its gravity. The blue cloud is direct evidence of dark matter that you can see for yourself.

While we haven’t directly detected dark matter, thanks to evidence like the Bullet Cluster we are certain it exists in some form, and we (fail to) see it everywhere. It can be thought of as the scaffolding of a galaxy, or even of larger cosmic structures. In our Solar System, the farther a planet is from the Sun, the slower it orbits. This occurs because most of the mass in our Solar System is concentrated at its center. The same would be expected of stars orbiting the center of a galaxy, as a galactic core holds far more visible mass than the halo. But this isn’t what we observe. Instead, stars on the outer edges of galaxies orbit at the same speed as those closer to the center. Without the additional gravity provided by dark matter in the halo, the velocity of these outer stars would be far too much for the galaxy to remain coherent, and the entire structure would be ripped apart. Instead, something invisible holds it all together.

At even larger scales, dark matter created in the Big Bang would have clumped together into blobs aligned along filaments. This dark matter would have formed the backbone of the cosmic web—a vast, lace-like structure so enormous that even galaxies are smaller than a grain of sand. The dark matter provides the gravitational pull that attracts gas and dust together to form galaxies in the first place.

Dark matter is a hypothetical solution to a very real problem. While not all scientists agree on what dark matter is, they do tend to agree that it likely exists in some form, whether that be one of our hypothesized forms, or something we can’t yet imagine. It is the very glue holding our universe together, and yet we know next to nothing about it. It is truly one of the greatest mysteries of our time.