This composite image shows the galaxy cluster 1E 0657-56, also known as the "bullet cluster", formed after the collision of two large clusters of galaxies. Hot gas detected by Chandra is seen as two pink clumps in the image and contains most of the "normal" matter in the two clusters. An optical image from Magellan and the Hubble Space Telescope shows galaxies in orange and white. The blue clumps show where most of the mass in the clusters is found, using a technique known as gravitational lensing. Most of the matter in the clusters (blue) is clearly separate from the normal matter (pink), giving direct evidence that nearly all of the matter in the clusters is dark. This result cannot be explained by modifying the laws of gravity.

95% of the Universe is made of dark energy and dark matter. But what is the difference?

For every grain of sand on Earth there are 10,000 shiny stars in the Universe. As overwhelming as this may seem, all the atoms in all these stars only make up roughly 5% of the Universe.

The remaining 95% comes from dark matter and dark energy. But what are dark matter and dark energy? Are they just two different words for the same thing? Or are they completely unrelated?

Stars, gas and all the matter we know as ordinary matter only makes up 5% the energy in the Universe. The remaining 95% are dark matter and dark energy. Credit: NASA/CXC/K.Divona.

Before we look closer at the two concepts, let us remember that normal visible matter covers all stars, gas and dust in the Universe. Everything we know around us made of atoms are included in those 5%.

Dark matter holds galaxies together

Dark matter was discovered some 60 years ago, when astronomer Vera Rubin together with her colleagues observed that the outer parts of spiral galaxies rotated faster than predicted by calculations.

By knowing the mass and extent of an object (in this case a galaxy) it is possible to calculate the gravity and thus know how this gravity affects its surrounding. The astronomers calculated the expected rotation speed of each galaxy (stippled line in the figure below) and observed that the speed was in fact much higher (yellow points in the figure below).

Stars are expected to move slower in the outer areas of a galaxy compared to close to the center, because the gravitational effect is lower in the outer areas. However, observations show that stars orbit the galaxy at much higher speed than expected. Astronomers speculate that this extra gravity comes from matter, we can not see: Dark matter. In this graph the y-axis (V) is the rotation speed and the x-axis (R) is the distance from the center. Credit: Wikimedia Commons.

Astronomers have not been able to observe any additional matter to account for this extra gravity. The observation of each galaxy’s rotation indicates that there must be something there to interact gravitationally with stars and give them the increased speed.

This something has therefore been given the name ‘dark matter’. Simply because it interacts via gravity and it does not shine a light.

DARK energy causes an accelerated expansion of the universe

An individual galaxy is roughly 0.0001 billion light years across, and the stars in a galaxy are held together by gravity. If we zoom out from this galactic system to the edge of the Universe (a scale of almost 14 billion light years) we see that the Universe is in fact expanding.

This means that as time goes by, the observable Universe increases in size. What is even more spectacular is that a few decades ago astronomers discovered, that the Universe is not only expanding – it is also accelerating its expansion.

As we know from classical physics, acceleration does not happen spontaneously. There must be something responsible that drives the acceleration, and dark energy is the proposed something that causes this acceleration.

The effect of dark energy is therefore discovered at a much larger scale than the effect of dark matter around individual galaxies.

Validated by the cosmic microwave background radiation

Currently, astronomers agree that dark matter accounts for 26% of the energy in the Universe and dark energy accounts for 69%. This leaves 5% for ordinary matter, i.e. 5% to include all atoms we know.

Dark matter and dark energy have remarkably specific fractions of the total Universe, considering that no one really knows what either of them are.

These specific fractions (69% dark energy, 26% dark matter and 5% normal matter) are the resulting parameters when reconstructing the WMAP spectrum. You can create your own Universe and try to estimate the fractions on NASA’s website.

Cover photo credit
X-ray: NASA/CXC/CfA/M.Markevitch et al.
Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.
Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.