# How Do We Know That Dark Matter Exists?

You have probably heard of the term “dark matter” a lot, the mysterious type of matter which no one can see but fills up 27% of the Universe. That’s really huge, isn’t it? Visible matter on the other hand (including the stars, the planets, you and me, etc.) accounts for only 5% of the Universe. So how did we confirm the fact that dark matter exists? There are specific ways.

## Galactic Clusters

In 1933, a Swiss astronomer, Fritz Zwicky was the first person to suspect the fact that dark matter exists. Mr. Zwicky was trying to find the mass of the galactic cluster using two different methods. The measurement was attempted from a concept that was based on the speed of the galaxies. There’s a layman’s explanation for this.

You have probably noticed kids on a merry-go-round having to hold on to something to avoid getting ejected. The same concept is somewhat applied in Zwicky’s findings. All galaxies are held together in a cluster by the gravitational force of matter that it contains. If there weren’t enough matter in the universe, the galaxies would scatter because enough force won’t be generated to hold the galaxies together in a cluster. He came to the conclusion that the galaxy has way more matter than the ones that were visible. That unknown and the invisible matter are therefore known as dark matter.

## Velocity curves

Vera Rubin, an American astronomer in 1970, measured the speed of stars in galaxies to a certain level of accuracy which was acceptable enough from the point of view of the scientific community. She noticed that stars in the spinning galaxy were rotating with roughly the same velocity. The distance from the galactic center doesn’t matter in this case. This theory is in direct violation of Kepler’s law.

According to Kepler’s law, a planet which is further away from the sun rotates slower than the one closer to the sun. Refer to curve A labeled in the graph below. Vera Rubin’s theory doesn’t approve of the law. According to her findings, the stars follow the curve B depicted in the graph below. This is like the stars aren’t rotating around the sun (the visible center of the universe). They are rotating around many unknown centers. All these centers are providing additional forces of gravitation. Thus, the theory of dark matter gained relevance yet again.

Image courtesy- PhilHibbs

## Gravitational lensing

This is another effective technique of detecting dark matter. Gravitational lensing is based on a specific way where a large concentration of matter (either visible or dark) create a gravitational force so strong that it becomes strong enough to distort space.

Remember the bending of light theory? Well, it’s part and parcel of the gravitational lensing theory.

Light behaves in the same manner as space. Meaning, it moves in a straight line. In presence of a huge object like a galaxy or a star, light tends to follow the curvature of that distorted space. The light will, therefore, bend when it passes close to a huge cluster of dark matter (refer to the image below).

## Cosmic Microwave Background

The existence of dark matter can also be confirmed from the study of CMB (cosmic microwave background). CMB is a form of cosmic background radiation which is part and parcel of observational cosmology and is considered to be the oldest light in the universe.

Take a look at the image below.

Map of the universe taken by the Planck satellite

The hotter spots on the map correspond to that of the dark matter.

So you see dark matter might actually stay in the dark but it does exist. This topic is highly interesting because of all the mysteries surrounding it. With that, we’ll bring this article to an end. Hope you had a good read.

### Sudipto Das

Sudipto writes technical and educational content periodically for wizert.com and backs it up with extensive research and relevant examples. He's an avid reader and a tech enthusiast at the same time with a little bit of “Arsenal Football Club” thrown in as well. He's got a B.Tech in Electronics and Instrumentation.