The first thing that is the matter with dark matter is that it is not “dark” at all. It’s invisible. It neither emits nor absorbs light.
The second thing that is the matter with dark matter is that the fate of mankind depends on something that science only “infers” to exist. This invisible, theoretical, dark matter holds the existence of the universe in its unseen “hands”. Scientific principles regarding gravity conclude that without this elusive dark matter every star, planet, and all humans as well, would go flying willy-nilly into outer space.
The third thing that is the matter with dark matter is that if it exists, it is then possible that dark matter creates a parallel, invisible world. All the happenings of another civilization could very well be happening right under our very noses and humankind is completely left out of the loop. How utterly curious. Could this parallel world have a cure for cancer? Does cancer even exist there? Are there political factions squabbling for power? Would such a world even need governance? What a fascinating idea.
These matters are why such a hubbub is made within the scientific community about dark matter. The simplest definition of dark matter is that it is nonluminous (dark, invisible) material that is hypothesized (scientifically assumed) to exist in space. It is thought that it can have different forms such as:
- Cold Dark Matter: particles that are slow moving when compared to the speed of light and interact weakly with ordinary matter and electromagnetic radiation
- Warm Dark Matter: particles with properties that could possibly be sterile neutrinos and/or gravitinos, and travel faster than cold dark matter but slower than hot dark matter
- Hot Dark Matter: (no, not an erotic vampire novel) high-energy particles, moving randomly, and do not interact with electromagnetic radiation
Dark matter is theorized to have been created soon after the Big Bang. Therefore understanding dark matter is critical to understanding and supporting the Big Bang theory. Scientists tend to gravitate toward the theory of the creation of the Universe through the building blocks of cold dark matter after the Big Bang. Structures would grow from the bottom up by smaller objects collapsing because of their own gravity. These collapsed structures would then merge and form larger objects with greater mass. Theorizing that the Universe evolved from cold dark matter collapsing and structural fragments merging resolves the questions of how individual galaxies formed.
Warm dark matter and hot dark matter alone could not hold up under scientific scrutiny as to being the original building blocks of the Universe. Although, it may very well have been a mixed bag of all three forms of dark matter creating structures that ultimately resulted in the Universe as we know it today, such a theory, the Mixed Dark Matter theory, is generally rejected.
The universe that is currently known to man consists of about five percent of matter that is classified as “ordinary”. That means that about five percent of the universe consists of matter with mass that is comprised of atoms, or ions, with a nucleus and protons and neutrons. Cosmologists call these “baryons”. This is the matter humans can see.
If ordinary matter only makes up about five percent of the universe, what is the remaining ninety-five percent made up of? About seventy percent is “dark energy”, or, a theoretical energy in the form of a repulsive force counteracting gravity which results in an accelerated expansion of the universe. Dark matter is thought to make up the balance. It sounds like a recipe straight out of Frankenstein’s laboratory: seven cups of dark energy, three cups of dark matter, and a splash of ordinary matter. Voila, Universe!
Detecting dark matter requires a whole new level of thinking. It does not absorb light. It does not emit light. It produces no detectable levels of electromagnetic radiation. If it’s invisible, and cannot be seen with a telescope, how do cosmologists and astronomers know it exists? Scientists infer the existence of dark matter. When astrophysicists measure the mass of large objects in space, such as stars, they discover discrepancies with regard to gravitational effects. When things just don’t add up, the scientists scratch their heads and ask, “Why do these heavenly bodies generate a gravitational effect that should actually be created by an object with greater mass?”
Questions such as these were being asked as early as 1932 when scientist Jan Oort, a Dutch astronomer, suggested dark matter was to blame for the orbital speed of the stars within the Milky Way galaxy. The following year Swiss astronomer, Fritz Zwicky, also believed dark matter was the culprit for the “missing mass” issue. However, it would take another thirty years before compelling evidence could assist the theory of dark matter in gaining ground in the scientific community.
In the 1960’s and 1970’s, American astronomer, Vera Rubin, was deeply entrenched in her controversial work on galaxy clusters. Working alongside Kent Ford, an astronomer and instrument maker, the pair used his spectrometer design to view the light spectrums of spiral galaxies. Their discovery is called the Rubin-Ford effect.
This phenomenon describes the movement of the Milky Way galaxy relative to sample galaxies. Rubin and Ford theorized that the difference in motion of these galaxies, compared to the Milky Way’s motion, was relative to cosmic microwave background radiation. Rubin then focused on studying the rotation curves of galaxies. This led to the discovery of discrepancies between predicted angular motion of galaxies and the actual observed motion of the galaxies.
The gravity of stars within rotating galaxies is what prevents these galaxies from flying apart. Such strong gravitational forces require immense mass. Rubin’s calculations revealed that such galaxies contained much more mass than could be accounted for by the stars they contained. Attempting to explain this discrepancy became known as the “galaxy rotation problem” and led to the conclusion that dark matter must then exist.
One of Rubin’s observations showed that as much as six times more “dark” mass existed in galaxies than ordinary matter. Her work was highly controversial at the time and continued to be studied, tested, debated and discussed. As more astronomers did their own studies with conclusions that supported Rubin’s assertions, it became well established within the scientific community that most galaxies are predominately “dark matter”.
The result of Rubin and Ford’s work has led to innovative methods of observing galaxies. One such method, gravitational lensing, was used to examine background objects within the Bullet Cluster in attempts to identify the presence of dark matter. Light bends as it travels away from the source to the observer. It is the mass of the observed object which causes the light to bend. The greater the mass, the stronger the gravitational field it creates, thus a greater degree of bending of the rays of light. When light is then bent to a degree greater than would be indicated by the known mass of the astronomical object, dark matter is then assumed to be at play to account for this mathematical anomaly.
Scientists have used gravitational microlensing to conduct large searches throughout the Milky Way galaxy. Astronomical evidence indicates that the universe contains much more matter than what is visible to mankind. Some scientists have even speculated that a parallel world is possible that consists of dark matter and can only interact with the universe as we know it through gravity.
When measuring the velocity of rotation as compared to the distance from the center of a spiral galaxy, such as the Milky Way galaxy, the mathematical discrepancy reveals that the cluster’s mass consists of very little of the ordinary matter objects that are visible. Scientists then suggest that dark matter is concentrated in a halo formation surrounding the visible matter. A dark matter parallel world could perhaps be found in the “halos” around astronomical objects. Since dark matter contains no atoms, like ordinary matter, it cannot interact with ordinary matter through electromagnetics. Dark matter contains no electrical charge. Hence, gravity is the only interactive relationship between dark matter and ordinary matter as the theory is understood at this time.
Spiral galaxies are not alone in containing dark matter. Studies conducted with gravitational lensing reveal that dark matter may very well be present in elliptical galaxies. Within dark halos that surround such galaxies, X-ray emissions indicate atmospheric extensions of hot gas which could support the existence of dark matter. Using X-ray emissions to estimate dark matter existence is achieved by measuring the energy and fluctuation of the X-rays. These measurements can be used to estimate the temperature and density of the gas producing the X-rays as well as the pressure of the gas. A profile of mass can be created by assuming that the gas pressure balances with the present gravity. Discrepancies would then be attributed to dark matter.
As with anything, there are, of course, exceptions to the rule. Globular clusters are thought to perhaps contain no dark matter. Cardiff University astronomers discovered galaxy VIRGOHI21 in 2005 and believe it to be made up entirely of dark matter and absent of any visible stars. So, there is diversity and oddities even amongst the stars.
Dark matter within our very own Milky Way galaxy is, apparently, “wimpy”. Every second of every day millions, perhaps even billions, of weakly interacting massive particles, also known as WIMPs, pass through this globe humans call home. Experiments of detection are vigorously underway searching for these invisible invaders. Because WIMPs do not interact with matter, it is thought that they can be detected by measuring energy and momentum discrepancies as they zip about, collide and annihilate each other. This is one of the studies conducted in supercolliders.
What does the discovery of dark matter mean for mankind? For the scientific community, it is simply another wonderful puzzle to be solved. For the regular person moving through life every day, it might mean a new awareness of the possibility of an invisible world right next to you. Average people who simply want to rise from a chair and cross the room may find themselves compelled to politely mutter the words, “Please excuse me.” These words may appear to be uttered to an empty room containing no one who needs their pardon begged. No, these people are not crazy and talking to themselves, they are simply considering that the room could contain invisible, dark matter co-habitants that find it very disturbing when a human walks right through them without even a, “How do you do?”