We live in on planet Earth, where the atmosphere is made of elements such as nitrogen, oxygen, and carbon dioxide and where its inhabitants, like us and all living things we see, are composed of mostly carbon-based compounds. Earth is, in turn, situated within a Solar System, where other rocky and gaseous planets rotate around one big ball of gas and elements, the Sun. Nuclear reactions occurring between different atoms in the Sun’s core are what create the heat we feel. This solar system is then in the galaxy, the Milky Way. From there, the Milky Way fits into the Local Group, and so on and so forth, until we get to the Universe. But what is the Universe made out of?
According to NASA’s website, the Universe is composed of something known as dark energy, dark matter and baryonic matter. Baryonic matter is normal matter, or ‘conventional’ matter. It is everything formed by protons and neutrons, a definition that can be stretched to include electrons. Dark energy, however, is something unknown. It is roughly described by the Hubblesite as “a force that increases the expansion of the universe”. Dark matter’s existence is only known because of indirect detection.
When looking at the composition of the universe, it is often viewed with the following percentages: approximately 68% dark energy, 27% dark matter, and less than 5% baryonic matter. Therefore, the part of the Universe we actually know about, is minuscule.
How do we know of the existence of dark matter?
Dark matter’s existence is inferred from many different observations. One of these comes from a phenomenon known as gravitational lensing, when light traveling from a light source to an observer gets distorted by a massive object within the light’s path. For example, when we look at a galaxy that is actually behind a galaxy cluster. The light from the galaxy is distorted by the massive galaxy cluster, and the image we see is distorted. The distortion is determined by the Theory of General Relativity and is dependent on the mass of the galaxy cluster, or any other massive object distorting the light rays.
However, after having performed calculations taking into account the mass of galaxy clusters for galaxies whose image is distorted, scientists have concluded that the matter observed within the galaxy superclusters does not account for the amount of distortion the light rays suffer. Therefore, there must be more unseen matter in these clusters. This leads to why dark matter cannot be detected with current methods. It does not emit electromagnetic radiation, so we have no way to observe it directly. Read more about gravitational lensing here (http://w.astro.berkeley.edu/~jcohn/lens.html ).
Candidates for Dark Matter
Even though scientists do not know what dark matter is, there have been many suggested candidates.
The hypothetical particle known as the Weakly Interacting Massive Particle (WIMP) is another suggestion. With its succes, this would become a new elementary particle.
MACHOs, or Massive Compact Halo Objects, are any objects that do not absorb or emit enough light to be detected, but are formed by normal matter, such as Brown Dwarfs. Brown Dwarfs are some of the lightest stars but are not very luminous, as they are too small to have nuclear fusion of hydrogen at their cores. To read more in detail, head over to Berkeley’s webpage.
Unfortunately, there are many things we do not fully understand about how the Universe works, and as we can see, scientists have barely begun to scratch the surface of the knowledge to be gained. What is the rest of the Universe made out of? This is yet to be discovered.
Edited by Briana Fannin