Dark Matter is theorized to be matter in the universe that doesn’t interact with light and the electromagnetic spectrum. This means that we can not see it with the naked eye, even though this matter may be all around us. Ultraviolet and Gamma Detectors that normally would allow us to see gasses in far away objects, such as nebulae, would also unable to spot dark matter.
Facts about Dark Matter
Galaxies should theoretically fly apart if they only consist of the visible matter that can currently be detected.
Dark Matter could potentially solve the galaxy / velocity problem.
The existence of dark matter is still theoretical.
Direct detection of dark matter is underway at a number of experimental facilities around the world.
Dark Matter could be a number of things, including MACHOs, WIMPs, SIMPs and Axions.
Does it even exist?
If we are unable to see dark matter, then why do scientists believe it exists? This is a great question! The theory of the existence of dark matter dates back to the 1920’s and 1930’s.
Astronomers who were studying galaxies and galaxy clusters at the time began to notice that these objects did not behave according to the laws of physics. When they viewed the motions and speeds of stars at the edges of local galaxies, they saw that the stars at the edges of the galaxies moved at approximately the same velocities as stars towards the center. This should not be possible!
If we look at the orbital velocities of the planets around our Sun, we quickly notice that planets close to the Sun have higher orbital velocities than those furthest from the Sun. Mercury, for example, has an orbital velocity of 47.4 km/s, whereas Neptune has an orbital velocity of 5.43 km/s. The astronomers viewing these galaxies found that stars at the edge had nearly the same orbital velocities as stars near the center, causing them to act in mysterious ways.
Many astronomers came to the conclusion that there must be a tremendous amount of matter inside the galaxies that was invisible or unseen. This unseen matter would hold the galaxies more compactly than the visible stuff alone could do. The other possibility was that our collective understanding of gravity was wrong. The debate about which possibility is correct still continues today!
From the 1930’s until now, astronomers and scientists have continued to utilize various methods and more sensitive equipment to attempt to confirm the probable existence of dark matter. However, its nature is still baffling, and its direct detection remains elusive to this day.
What are quarks?
Just as atoms are tiny building blocks of matter, quarks are even smaller building blocks of atoms and other particles. Any particle that has quarks is called a hadron.
There are several types of hadrons. Baryons contain three quarks, while mesons contain 2 quarks. Again, both baryons and mesons are types of hadrons.
Baryonic vs. Non-Baryonic Matter
Let’s take a deeper look at baryonic vs. non-baryonic matter.
Baryonic matter is defined as objects in our universe that have atoms and/or atomic structure as its building blocks. Atoms, if you recall, have a nucleus made of protons and neutrons in a nucleus, and electrons that surround or orbit the nucleus. Because protons and neutrons have three quarks, they are referred to as baryons. This is why matter formed from atoms is called baryonic matter.
Most matter that we have knowledge of and interact with is baryonic matter. Examples of baryonic matter include comets, asteroids, stars, planets and the things we see around us on Earth.
Non-Baryonic matter is just the opposite, and is defined as matter and particles that do not have standard atomic structure. Mesons are examples of non-baryonic matter – they are particles that only have two quarks.
Some examples of non-baryonic matter include WIMPS, or Weak Interacting Massive Particles and Neutrinos. Electrons are technically non-baryonic matter as well, but often get lumped together with baryonic as they are part of atoms.
Possible Dark Matter Candidates
So what does all of this have to do with dark matter? Well, scientists think that dark matter could be any one of a number of things, many of which aren’t baryonic matter. This is all still theoretical and still very much debated! Below are a few of the theorized possibilities:
MACHOs – MACHOs stands for Massive Astrophysical Compact Halo Objects. Collapsed remnants of stars, known as neutron stars, and brown dwarf stars, are examples of MACHOs. These objects do not emit light, so we can not easily detect them. They could potentially “hide” inside galaxies, and some of them are extremely dense. These would all likely be types of baryonic matter.
Scientists once thought that if enough MACHOs were inside galaxies, that this could explain the galaxy velocity question. These massive objects, however, have eluded detection, and many scientists now believe that MACHOs should no longer be considered as viable candidates for the dark matter needed to solve the velocity problem.
WIMPs – WIMPs, or Weakly Interacting Massive Particles, are theorized to be large, non-baryonic, slow moving particles. They are also thought to be neutral – meaning they would have no charge like a neutron except with two quarks instead of three.
Scientists aren’t exactly sure how big they could be in comparison to standard particles such as protons or neutrons. They only believe that they aren’t standard particles. Some physicists now think, after decades of searching, that WIMPs may also need to be ruled out as a solution.
SIMPs – SIMPs stands for Strongly Interacting Massive Particles. They are theorized to be particles that are composed of two quarks, rather than the three quarks that protons and neutrons consist of.These theoretical particles would interact strongly by gravitational force, but weakly by electromagnetic force. This would still make them difficult to detect by traditional methods.
Experiments to Detect Dark Matter
A number of experiments, which often involve very expensive machinery and materials, have been developed in an attempt to detect dark matter. COSINE-100 is one such experiment, located in an underground facility in South Korea. COSINE-100 utilizes thallium-doped sodium iodide (NaI) crystals lowered into liquid scintillator to try to find WIMPs.
Another experiment, called PICO, attempts to detect tiny bubbles that could be created by WIMPs. This experiment uses detectors to try and locate bubbles in super hot liquid. PICO labs are buried deep underground, to try and limit how many particles can reach the liquid. If a bubble does form, there is a chance that a dark matter particle caused it.