What is Dark Matter? - Theory & Energy

Instructor: Richard Cardenas

Richard Cardenas has taught Physics for 15 years. He has a Ph.D. in Physics with a focus on Biological Physics.

In this lesson, you will learn what dark matter is and the latest theories about the origin of dark matter. You will also learn about dark energy and find out if dark matter and dark energy are related to each other.

Definition

What we can see in the sky either with the naked eye or with optical instruments is only a small fraction of what we know as the universe. Why? The rest of the universe does not emit any kind of light in any spectrum that we can detect; however, the presence of this matter and energy is detected by the effect it has on other matter in the universe. When light from stars passes through a region with dark matter, the light gets distorted and bends, making the star appear in a different location in space. This is called gravitational lensing, an effect usually associated with normal matter. Scientists have detected gravitational lensing in regions where there is no normal matter, leading them to conclude that dark matter is present. This material comes in two forms: dark matter and dark energy. Dark matter and dark energy are the big scientific mysteries in cosmology, the study of the origin of the universe, among other things.

In 2008, after five years of collecting data from space, scientists developed a theory that details the composition of the universe. According to the data, nearly 95% of the universe is a combination of dark matter and dark energy, with 68% dark energy and 27% dark matter. Only 5% of the universe is normal matter. Normal matter is baryonic, made up of protons and neutrons. Normal matter also absorbs or emits radiation and therefore can be detected. So why is the dark matter dark? It is called dark because it is non-luminous matter: it does not emit or absorb any type of radiation that we are currently able to detect. Therefore, any type of traditional instrumentation would just see it as empty space. What is dark energy? Current theories speculate that dark energy is a property of space. Empty space is not just nothing, something Albert Einstein recognized first. Dark energy is therefore thought of as the energy of empty space.

What Is Dark Matter?

The idea of dark matter made its entrance when theoretical scientists were trying to create a model of the composition of the universe. Up until 1998, there did not seem to be enough mass in the universe to fit with the current theories that suggested that about 32% of the universe should be made up of matter. More recent theories suggest that the universe is composed of 5% matter, so what about the remaining 27%? To fix this discrepancy, scientists developed the idea of dark matter.

What Dark Matter is Not

Describing what exactly dark matter is can be difficult; it's actually easier to explain what it is not. To exemplify, matter is made up of baryons, a group of particles that includes protons and neutrons. But, dark matter is not made up of baryons. Why? Baryons produce baryonic clouds that absorb radiation when radiation passes through them. This absorption would be detectable if dark matter was indeed comprised of baryons. Dark matter is not antimatter either. If it were, then we would be able to detect the gamma radiation emitted when matter and antimatter annihilate each other. Dark matter is not a series of black holes either. Black holes cause gravitational lensing, a phenomena that causes light from distant stars to bend, causing them to appear as if they are originating from a different location in the universe. There is just not enough gravitational lensing observed to make up 27% of the universe.

As you can see, it is easier to say what is not dark matter than what is dark matter. There are a few candidates out there that can be considered dark matter. One of the initial candidates were called massive compact halo objects, or MACHOs. MACHOs are small pieces of baryonic matter (much smaller than a star) that just drift in interstellar space. They do not emit any light; therefore, they would be difficult to detect. However, more recent theories suggest that MACHOs are not abundant enough to account for the amount of dark matter in the universe. Also, there are not enough baryons to make up dark matter if dark matter was baryonic. Since dark matter is not baryonic, then MACHOs have been discounted as candidates for dark matter.

Possible Dark Matter Candidates

Current theories suggest that weakly interacting massive particles, or WIMPs, are prime candidates for dark matter. WIMPs are subatomic particles not made up of ordinary matter. They have mass and they weakly interact with matter, meaning they mostly pass through ordinary matter. Three particles are proposed to be WIMPs: neutrinos, axions, and neutralinos.

Neutrinos were created to help with the theory of particle interactions. They were initially theorized to be massless but have since been discovered to have mass. However, the neutrino has such a small mass that neutrinos alone could not contribute 100% to the total mass of dark matter. Axions are particles that were also proposed to fill a gap in a theory: the absence of an electric dipole moment of a neutron. They too have a small mass and would not have been created in large quantities during the 'big bang.' Therefore, dark matter could not be completely composed of axions. Neutralinos are particles proposed as part of a theory that tries to unify all of the fundamental forces of nature (also known as supersymmetry). These particles are very massive particles, having masses of up to 5000 times the mass of the proton. These three particles combined are currently the prime candidates for dark matter.

What Is Dark Energy?

The universe is mostly made up of dark energy. In fact, about 68% of the universe is dark energy, according to current theories. Dark energy is proposed to be a property of space. There are four possible explanations for dark energy. Albert Einstein was the first to consider the concept of dark energy in his theory of gravity, where he proposed that empty space can possess its own energy. This energy would be diluted as the universe expands, and as the universe expands, there will be more space and hence more of this dark energy. This energy would also cause the universe to expand faster and faster. Quantum physics proposes an alternative explanation of dark energy. In this version, so-called virtual particles appear and disappear in empty space all the time. However, calculation of this energy came out way too high.

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