When we look around us, it can seem like the universe is made entirely of matter. Matter is anything that has mass and takes up space. We can see matter almost everywhere. However, although the universe does contain matter, it makes up only a very small part of what the universe is made of.
The matter we can see is called visible or normal matter. It makes up only about 5% of the universe.
So what makes up the rest?
Most of the universe is made of things we cannot see. About 27% is dark matter, and around 68% is dark energy.
- Dark Matter
Dark matter is difficult to understand. It refers to material in space that we cannot see directly. Even though we cannot see dark matter, we know it is there because of the effects it has on other objects.
Dark matter must have mass, and therefore it must also produce gravity. One way we detect dark matter is by studying how spiral galaxies rotate. Observations show that galaxies contain far more mass than we can account for using visible matter alone. In some galaxies, as much as 90% of the total mass must be dark matter.
Scientists still do not know what dark matter is made of. Some ideas suggest it could be made from faint objects, such as long-dead stars, unseen planets, or black holes. However, these objects are made of particles that we can usually detect.
Other ideas suggest that dark matter may be made of new types of particles that have not yet been discovered.
Some scientists have also suggested that the laws of physics may behave differently over very large distances.
Research continues to investigate why measurements based on light do not match measurements based on gravity.
- Dark Energy
We know that the Universe is expanding. When astronomers observe distant galaxies, they see that these galaxies are moving away from us. The more distant a galaxy is, the faster it appears to be moving away. This tells us that the expansion of the universe is speeding up.
Astronomers do not yet know why this is happening.
One possible explanation is a force called dark energy, which is thought to be pushing space itself apart.
In physics, energy and mass can be thought of as two forms of the same thing. This idea is described by Einstein’s equation E = mc2. It shows that mass can be converted into energy, and energy can be converted into mass.
For this reason, dark energy is included when we describe the overall composition of the Universe. Dark energy remains one of the biggest mysteries in astronomy. Scientists are still working to understand what it is and how it affects the Universe.