# Difference Between Dark Matter and Dark Energy

## Main Difference – Dark Matter vs. Dark Energy

Understanding the dark matter and dark energy is one of the key mysteries in science. The existence of both dark matter and dark energy is supported by a number of different observations. However, it is still not known how dark matter and dark energy originates, or what they are composed of. The main difference between dark matter and dark energy is that dark matter interacts via gravity and tries to bring matter together, whereas dark energy accelerates the expansion of the universe, thereby pushing matter apart.

## What is Dark Matter

In the early 1930s Fritz Zwicky, a Swiss astronomer, was studying how galaxies were moving in galaxy clusters. He could calculate the mass of a galaxy using two methods. Firstly, by looking at the motion of galaxies, he could deduce the gravitational forces between the galaxies and determine how much mass should be present. Secondly, he could measure the brightness of galaxies and deduce how much matter should be present. His results showed a discrepancy: when he used the motion to calculate mass, he came up with a much larger value than when he used light to measure mass. To explain this, Zwicky believed there must be some other invisible “dark” matter that could not be accounted for by the light.

For the next four decades, not much serious research was done regarding this mystery. In the 1970s Vera Rubin, who was studying how fast the stars were moving around the centre of a galaxy, noticed that stars further away from the centre were moving around with faster speeds than they should have. She, too, concluded that there must be some invisible matter in a galaxy that can account for this behaviour. The image below summarises her findings:

A galaxy rotation curve – the graph shows the speed with which the stars move around in a galaxy, as a function of the distance of the star from the centre of the galaxy. The solid line shows the observed result, while the dotted line shows the result that was expected when only visible mass (i.e. ordinary matter) is considered.

Another compelling case for the existence of dark matter comes from gravitational lensing. According to the theory of relativity, when light travels past massive objects, the path of light gets curved. As a result, distant galaxies can appear distorted.

Gravitational lensing distorts the images of distant galaxies

The Bullet Cluster consists of two galaxies moving past each other after having collided. An image of the bullet cluster is shown below. We can determine where the ordinary matter is in this galaxy, by looking at x-rays emitted by gases. The pink regions on the image show where the ordinary matter is concentrated. However, by studying gravitational lensing effects produced by the Bullet Cluster, most of the mass is found to be concentrated in the regions shown in blue.

The Bullet Cluster: The regions in pink show where the ordinary (visible) matter is most concentrated. The blue regions show where most mass should be present from measurements of gravitational lensing.

This is a strong indication that dark matter exists. When the galaxies collided, dark matter particles should be able to move past each other relatively quickly because they only interact strongly via gravity. Ordinary matter interact much more with each other (with electromagnetic forces for example). Therefore, it takes much longer for ordinary matter to go past each other. This explains why the pink areas are present towards the centre of the cluster.

## What is Dark Energy

Light from the stars that are moving away from us become redshifted. i.e. when we look at the light, it appears redder than it should be. In the late 1920s, Edwin Hubble realized that more distance stars always have redshifts, showing that the universe was expanding. In the late 1990s, measurements of distances and the speeds from stars even further away using type Ia supernovae revealed that the universe was actually expanding at an accelerated rate. This type of acceleration cannot originate from ordinary matter or dark matter because they interact via gravity and  should, in fact, work against the expansion of the universe. Therefore, dark energy is thought to be responsible for accelerating the expansion.

Another piece of evidence for dark energy comes from the small fluctuations present in the cosmic microwave background (CMB) radiation. These fluctuations show that the universe is close to being “flat”. The mass-energy density of ordinary matter in the universe is nowhere near enough to make it flat. Even if we include dark matter, the density still falls short. This can be reconciled if we take the rest of the mass-energy to come from dark energy. From cosmic microwave background measurements made by the Wilkinson Microwave Anisotropy Probe (WMAP), current estimates for the composition of mass-energy in the universe are as follows:

The mass-energy content of the universe, compiled from WMAP data (NASA)

It should be mentioned that the presence of dark matter and dark energy is not accepted by some scientists. Instead, they support alternative theories for describing the effects that we attribute to dark matter and dark energy. These theories often add modifications to the theory of relativity in order to make explanations. However, support for such alternative explanations is dwindling.

## Difference Between Dark Matter and Dark Energy

### Effect on Matter

Dark matter can interact via gravity so it contributes to bringing matter together.

Dark energy causes the universe to expand at an accelerated rate, causing matter to move apart.

### Presence

Dark matter is not thought to be distributed uniformly.

Dark energy is thought to be distributed evenly throughout the universe.

Image Courtesy

“Expected (A) and observed (B) star velocities as a function of distance from the galactic center. Created as a replacement for File:newtonianfig2.pngat English Wikipedia.” by PhilHibbs (Own work in Inkscape 0.42) [CC BY-SA 3.0], via Wikimedia Commons

“What’s large and blue and can wrap itself around an entire galaxy? A gravitational lens mirage…” by Lensshoe_hubble.jpg: ESA/Hubble & NASA (Lensshoe_hubble.jpg) [Public Domain], via Wikimedia Commons

“Composite image showing the galaxy cluster 1E 0657-56, better known as bullet cluster…” by NASA/CXC/M. Weiss (Chandra X-Ray Observatory: 1E 0657-56) [Public Domain], via Wikimedia Commons