DARK MATTER

What is Dark Matter?

• Dark matter is a mysterious substance that composes about 27% of the makeup of the universe.
• It isn’t ordinary atoms – the building blocks of our own bodies and all we see around us.
  • Atoms make up only somewhere around 5% of the universe, according to a cosmological model called the Lambda Cold Dark Matter Model
• Again, Dark matter isn’t the same thing as dark energy, which makes up some 68% of the universe.
• Dark matter is invisible; it doesn’t emit, reflect or absorb light or any type of electromagnetic radiation such as X-rays or radio waves.
• Thus, dark matter is undetectable directly. This is because all our observations of the universe, involve capturing electromagnetic radiation in our telescopes. The exception is detection of gravitational waves.

Properties of Dark Matter

Optically Dark (Dissipationless)

• Dark matter does not shine. Thus, dark matter particles must have very weak electromagnetic interactions.
• An important consequence of this is that the dark matter cannot cool by radiating photons, and thus will not collapse to the center of galaxies as the baryons do, by radiating their energy away electromagnetically.
• In other words, the dark matter is very nearly dissipationless.

Baryons and mesons are both hadrons. Baryons are heavy subatomic particles that are made up of three quarks. Protons and neutrons, as well as other particles, are baryons. Mesons on the other hand are composite particles made of one quark and one antiquark. A quark is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons. The most stable form of Hadrons are protons and neutrons, the components of atomic nuclei. In particle physics, a hadron is a subatomic composite particle made of two or more quarks held together by the strong force in a similar way as molecules are held together by the electromagnetic force.

Collisionless

• In addition to not interacting with “light” matter, the dark matter is collisionless as well.

Dark matter is not clouds of normal matter

• Normal matter particles are called baryons. If dark matter were composed of baryons it would be detectable through reflected light.

Dark matter is not antimatter

• Antimatter annihilates matter on contact, producing gamma rays. Astronomers do not detect them.

Dark matter is not black holes

• Black holes are gravity lenses that bend light. Astronomers do not see enough lensing events to account for the amount of dark matter that must exist.

Dark Matter interact with ordinary matter

• It exhibits measurable gravitational effects on large structures in the universe such as galaxies and galaxy clusters.
• Because of this, astronomers are able to make maps of the distribution of dark matter in the universe, even though they cannot see it directly.

Dark matter binds galaxies together

• Dark matter exerts ‘gravitational force’, meaning that it draws other matter towards it.
• And there’s so much dark matter that its gravitational force is enough to hold entire galaxies – like our own Milky Way – together.
• That’s why dark matter is often likened to a giant spider’s web, meshing galaxies in place.

Dark matter is cold in nature

What is Dark Matter made of?

• Decades out of the gate, scientists still don’t know what makes up the bulk of the universe’s matter. But they have some strong contenders.

 

Particle: Weakly interacting massive particles (WIMPs)

• Weakly Interacting Massive Particles, or WIMPs, represent one hypothesized class of particles to explain dark matter.
• They are heavy, electromagnetically neutral subatomic particles.
• They neither absorb nor emit light and don't interact strongly with other particles.
• But when they encounter each other, they annihilate and make gamma rays.

Particle: Axion

• A hypothetical subatomic particle of low mass and energy that is postulated to exist because of certain properties of the strong force.

Particle: Sterile neutrino

• Sterile neutrinos (or inert neutrinos) are hypothetical particles (neutral leptons – neutrinos) that interact only via gravity and do not interact via any of the fundamental interactions of the Standard Model.
• The term sterile neutrino is used to distinguish them from the known active neutrinos in the Standard Model.

The Standard Model explains how the basic building blocks of matter interact, governed by four fundamental forces.

Particle: Strongly interacting massive particles (SIMPs)

• Strongly interacting massive particles (SIMPs) are hypothetical particles that interact strongly between themselves and weakly with ordinary matter.

How do we detect Dark Matter?

• Dark matter does interact with ordinary matter.
• But it exhibits measurable gravitational effects on large structures in the universe such as galaxies and galaxy clusters.
• Because of this, astronomers are able to make maps of the distribution of dark matter in the universe, even though they cannot see it directly.

MACHOS - Massive compact halo object

• A massive astrophysical compact halo object (MACHO) is any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos.
• A MACHO is a body that emits little or no radiation and drifts through interstellar space unassociated with any planetary system (and may or may not be composed of normal baryonic matter).
• Since MACHOs are not luminous, they are hard to detect.
• MACHO candidates include black holes or neutron stars as well as brown dwarfs and unassociated planets.
• White dwarfs and very faint red dwarfs have also been proposed as candidate MACHOs.