In order to understand what dark matter is, we must first define what it isn’t.

Dark matter is not the same as dark energy. As previously discussed, dark energy is an elusive and inexplicable repulsive force acting on the entirety of space to accelerate its expansion. It has no mass, and interacts with no other forces in the known universe. Dark matter on the other hand has gravitational properties, thus it attracts, rather than repels.

Dark matter also is not ordinary matter. Ordinary matter, the stuff that makes up stars and planets and people, is the type of matter that interacts with the all of the fundamental forces of nature. Because of it’s interaction with the electromagnetic spectrum, we can see light it produces or reflects. Dark matter does not interact with or produce electromagnetic radiation, so we cannot detect it directly. It also does not physically interact with ordinary matter, which is why we can’t “feel” dark matter, even though it is all around us.

So How do we know it exists?

Expected (A) Orbital velocities of Milky Way stars vs Observed (B) orbital velocities.

If we can’t see or feel dark matter, why do we think such a thing exists? The answer lies in the one thing dark matter does interact with – gravity.

In the early 1930’s astronomers started calculating the orbital speeds of stars in the milky way galaxy. If only ordinary matter existed in the galaxy, one would’ve expected the center to be spinning significantly faster than the outer edges, much like water down a drain. However they discovered the stars on the outer edge were rotating with similar orbital speeds to the center – almost like a Frisbee. To account for this strange behavior, the idea of dark matter was proposed.

Dark matter is thought to be some type of yet-undiscovered particle that interacts heavily with gravitational fields, like ordinary matter. However because it cannot interact with light, we have no way of observing it directly. Thus, we can only determine its existence based on its gravitational effects on visible ordinary matter. It is believed there is a massive amount of dark matter permeating the Milky Way galaxy, and all galaxies, which accounts for the additional rotational velocities observed. Dark matter is thought to account for approximately 84.5% of the total matter in the universe.

CDMS Detection crystal

WIMPs and Experimental Detection

The prevailing theory of dark matter is the concept of WIMPs – Weakly Interacting Massive Particles. These particles are thought to interact with ordinary matter only through gravity and the weak nuclear force. Scientists are attempting to detect WIMPs both directly and indirectly. If they do in fact interact with ordinary matter via the weak force, there is a chance a collision with the nucleus of an atom could be detected directly. There are also numerous theories for how WIMPs could be detected indirectly from high mass celestial objects such as galaxy centers and the sun. Indirect detection involves looking for excessively high gamma rays or neutrinos that cannot be explained with the standard model of particle physics.

CDMS

One of the primary direct detection methods of WIMPs is known as CDMS (Cryogenic Dark Matter Search) uses hockey puck sized detectors of germanium and silicon crystals. These detectors are kept far underground to shield them from other sources of subatomic particle and radiation interactions, and are supercooled to just a fraction of a degree above absolute zero. The most sensitive detector is known as SuperCDMS, located in the Soudan Mine in Minnesota. Recently the DOE Office of High Energy Physics and the NSF have announced funding for a new CDMS project, known as SuperCDMS SNOLAB, which will be completed in 2018 at an underground lab in Ontario, Canada. SuperCDMS SNOLAB will be nearly three times as deep as the Soudan lab, making it much more sensitive to WIMP interactions.

The LUX Xenon Detector is 4800 feet below South Dakota

LUX

Another direct detection experiment is known as the Large Underground Xenon experiment (LUX), located at the Sanford Underground Laboratory in Lead, South Dakota. The LUX Operates similarly to CDMS, but instead of using supercooled crystals, it uses liquid Xenon. The principal is that if a WIMP interacts with the nucleus of the Xenon atom, it will produce both a photon of light as well as an escaping electron, which can be detected.
As of now, scientists are still hunting for a definite WIMP detection, as all of the possible detections have yet to be confirmed. In the coming years as detectors get increasingly sensitive, we will have a better picture of whether or not the WIMP model of dark matter is accurate.

To understand what is one of the most puzzling and fascinating forces in modern astrophysics, we first must understand just what exactly the universe is made of.

Universal energy breakdown

Einstein gave us the famous equation e=mc² , which essentially means the energy of a system (e) equals the mass of the system (m) multiplied by the square of the speed of light (c). Therefore, matter with mass can be mathematically converted into pure energy (and vice versa) while maintaining the law of Conservation of Energy.

When thinking about matter this way, we can then directly compare the amount of matter to the amount of energy in the universe. What was discovered was astounding. Approximately five percent of the universe is physical matter we can see. Every single atom in the universe, including every star, planet, and human, makes up only four percent of the total energy within the universe.

Another 27% of the total universe energy can be attributed to dark matter. Dark matter is matter that we can detect by means of gravity, but cannot witness it directly. It has similar properties to regular matter, except it does not interact directly with the electromagnetic spectrum – thus we cannot see it.

The remaining 68% of the universe is made up of dark energy.

Albert Einstein and Edwin Hubble

A History Lesson in Dark Energy

In 1917, Albert Einstein modified some of his General Relativity calculations to account for something he referred to as the “Cosmological Constant”. This was a mysterious and universal force he deemed was necessary to keep the universe from collapsing on itself by the force of its own gravity. Einstein was under the impression the universe was static, and therefore needed some explanation for why the observable matter was not forcing the universe to contract.

In 1929 Astronomer Edwin Hubble discovered all distant galaxies are moving away from us at a relatively uniform rate relative to their distance, which shattered the idea of a static universe. The cosmological constant was removed from Einstein’s equations, as it was no longer needed to explain why the universe was not collapsing. Einstein was quoted as referring to the Cosmological Constant as his “biggest blunder.”

Einstein would be vindicated almost 70 years later, when modern technology discovered distant galaxies were not only receding from us, but actually accelerating as they got further away. No calculations to that point could account for such a discovery, so Einstein’s cosmological constant was revisited.

So what is Dark Energy?

Illustration of the expanding universe

In a short answer, we don’t know. Like dark matter, it does not interact with regular matter or the electromagnetic spectrum, so we have no way of detecting it directly. All we know is the universe is expanding at an accelerated rate, which should not happen if it was made up of only matter and dark matter. One could think of dark energy as a “anti-gravity” force that pushes everything apart. In a sense, space itself is stretching due to the force of dark energy.

There are a few proposed explanations for the existence of dark energy, but none have been confirmed. One option is the idea that space itself has this associated expansion energy locked within it, even in a complete vacuum. This is the modern interpretation of Einstein’s Cosmological Constant. As this energy pushes outward to expand the space, more energy is created because it is proportional to the space itself.

Another alternative is found in Quantum Field Theory. Vacuum energy is the concept of “virtual particles” that temporarily exist in entirely empty space. These virtual particles are made up of perfectly opposite pairs that briefly flash into existence then disappear when they collide. The energy resulting in this annihilation could explain dark energy’s origins.

A third explanation is the concept of “quintessence”. This theory states dark energy could actually be a fifth type of fundamental force in addition to gravity, electromagnetic, strong nuclear, and weak nuclear. It is different than other explanations for dark energy in that it states the strength of the force has changed over time. Particularly about 10 billion years ago, it increased in strength to begin forcing the universe into its current accelerated expansion.

Implications of Dark Energy and an Expanding Universe

Regardless of the exact nature of dark energy, we are fairly certain the universe will keep accelerating in its expansion. This has some interesting consequences. As the space between distant galaxies and Earth grows, the galaxies will eventually recede at a rate faster than that of the speed of light. Thus, light emitted from those galaxies will never reach us – effectively removing them from our observable universe.

Additionally if the dark energy remains constant, there is a point at which it will reach divergent expansion, where the other universal forces are overpowered. In this scenario, all visible matter will be torn apart down to the very atoms, in what is known as the “Big Rip”

Where did the universe come from? Perhaps one of the most puzzling and inspiring questions humanity has ever posed. Countless religions, philosophers, and scientists have tried to answer this question, often at odds with one another. In the last century, scientific discoveries have finally given us a clue as to how old the universe is, what the early universe was like, and ultimately, what happened at the moment of creation.

The Big Bang Theory

Generally speaking, the Big Bang Theory is the overwhelmingly accepted concept for the creation of the universe. It essentially states that all of the matter and energy currently in the universe was at one time located in a single point of space – with infinite density – in what is known as a singularity. For reasons we are still not quite sure, the singularity exploded and sent pure energy out in all directions. As the early universe cooled, the energy began forming the earliest forms of matter, including protons, neutrons, and electrons. Over time, these particles formed the first elements, primarily Hydrogen and Helium. As the universe continued to expand, gravity took over and these elements formed the first stars. We can now say with relative certainty that the universe is about 13.8 billion years old.

The Four Pillars

Evidence for the Big Bang Theory comes from four distinct but related observations of our universe, which all directly support the theory’s key concepts.

Big Bang Neucleosynthesis

As mentioned, the early universe formed the first elements, a majority of which was Hydrogen, along with some Helium. These are the three lightest elements in the periodic table. We would expect to see about 75% of the early universe made up of Hydrogen, about 25% Helium, and about .01% of other trace elements such as Lithium. Upon observation of the universe as a whole, we find the ratios of these elements in the exact ratio we would expect, which is a strong indication our understanding of how early matter formed is correct.

Cosmic Microwave Background Radiation (CMBR)

WMAP visual representation of the CMBR

In the 1960’s Bell Labs built a radio antenna originally designed to communicate with early satellites. After its need was exhausted, it was turned over to two scientists, Arno Penzias and Robert Wilson took over use of the horn-shaped dish. When attempting to study radio signals from distant galaxies, they noticed no matter what direction they pointed the antenna, a weak but consistent electromagnetic signal was being received.

This signal is now known as the Cosmic Microwave Background Radiation. Shortly after the big bang, calculations indicate that a tremendous amount of radiation would have been emitted in all directions. Billions of years later, this intense radiation has settled into a faint microwave radiation, only a few degrees above absolute zero. Since Wilson and Penzias’ discovery matched exactly the prediction radiation, it became another strong support for the Big Bang model of the early universe.

Expansion and Hubble’s Law

One important requirement of the universe in order to support the Big Bang Theory is that if the universe started at a single point, and we now exist inside of it, it must have expanded. Based on the calculations of the early universe, we would expect the universe to be very large right now, and still expanding in all directions.

Representation of Red Shifting light from an object moving away from us

This was ultimately confirmed in 1929 by astronomer Edwin Hubble. Hubble discovered that distant galaxies (at the time called nebulae, because they were considered still a part of the Milky Way Galaxy), were receding from us. He determined this by analyzing the light emitted from these galaxies, and determined it was slightly stretched out from what we would expect. In the same way that sound can sound higher and lower as a moving vehicle passes you with the Doppler Effect, light waves can also compress and expand based on the velocity of the source. In all directions, distant galaxies seemed to be shifted towards the red end of the spectrum, hence the term Red Shift.

The Red Shift of galaxies in all directions indicates that the entire universe is expanding, and in almost all cases galaxies are spreading apart from one another. It stands to reason that if we play back the universe in reverse, all of these galaxies would at one point in time come from the same location – The Big Bang.

Galactic Structure and Evolution

Hubble’s Galactic Evolution

The final and most recent observable support for the Big Bang is how galaxies form over time. A valuable and important tool we have when studying the universe is the fact that light travels at a finite speed. This means light emitted from distant galaxies will not reach us for billions of years. Thus, we can look at the most distant observable galaxies and see what they looked like billions of years ago, in the early universe.

What we see is a very different picture of what we currently see in nearby galaxies. Early galaxies are simple in their structure, essentially “blobs” of young stars. Galaxies such as the Milky Way have ornate spiral structures, which have formed over billions of years. We also see, as we look back into the more recent past, that larger structure of galaxy clusters start to form which did not exist in the early universe. This is strong evidence that the complexity of the universe is increasing, meaning the universe must have at some point been much simpler than it is now.