The Dark Side
The mysterious dark matter is believed to consist of exotic non-atomic particles that do not interact with electromagnetic radiation. This exotic material only dances with so-called “ordinary” atomic (baryonic) matter by way of the force of gravity. According to the Standard Model of Cosmology, the Universe is composed of approximately 4.9% “ordinary” atomic matter, 26.8% dark matter, and a whopping 68.3% dark energy. Indeed, the dark energy, which makes up most of the Universe, is an even greater mystery than the dark matter. The most widely accepted explanation for the dark energy proposes that it is a property of space itself, and it is causing the Universe to accelerate in its expansion towards its own “heat death”. As the Universe speeds up in its expansion, it grows ever colder and colder; larger and larger–doomed to become an enormous frigid expanse, as its fires flicker out like a dying candle flame.
The very badly misnamed “ordinary” atomic matter is actually quite extraordinary. Even though atomic matter is clearly the runt of the cosmic litter of three, it accounts for all of the elements listed in the familiar Periodic Table. The atomic elements create the world that we are most familiar with, and that we can experience with our Earth-evolved senses. Even though atomic matter accounts for only a relatively small fraction of the Cosmos, it is what brought life into it. The iron in your blood, the calcium in your bones, the water that you drink, the sand that you walk upon are all composed of so-called “ordinary” atomic matter. Most of the atomic elements were formed within the searing-hot, nuclear-fusing furnaces of the Universe’s myriad stars. The Big Bang birth of the Universe, thought to have occurred almost 14 billion years ago, only manufactured the lightest of atomic elements–hydrogen, helium, and traces of lithium. The stars produced the rest in their incredibly hot cores, starting with the hydrogen and helium produced in the Big Bang, and then creating increasingly heavier and heavier atomic elements all the way up to iron by way of the process of nuclear fusion. However, the heaviest atomic elements of all–such as uranium and gold–were produced when a massive star blew itself up in a supernova explosion. These fiery, brilliant stellar blasts hurl freshly forged heavy atomic elements out into the space between stars–all manufactured in the searing-hot heart of the progenitor massive star, or in its explosive death throes.
According to the Standard Model for the formation of the large-scale structure of the Universe, exotic particles of the non-atomic dark matter at first performed a gravitational ballet with one another, thus constructing a crowded region of space, termed the dark matter halo. Gradually, the invisible primordial halos composed of the dark stuff snatched up clouds of pristine hydrogen gas. Hydrogen is both the most abundant, as well as the lightest, atomic element in the Universe. As a result, galaxies and their population of shimmering stars, emerged out of this primordial darkness.
Vast, swirling, and shapeless clouds of opaque pristine ancient gases gathered together in the primeval darkness. The clouds then somersaulted down into the secretive hearts of the strange halos of the dark matter. As time marched onward in the direction of the universal expansion, the very first generation of baby stars were born. Then, the newly lit fires of the first stars raged brilliantly within the first ancient galaxies that served the important function of being primordial stellar cradles.
Even though dark matter cannot be seen, it is generally thought to exist because of the very important discrepancies that scientists have observed between the mass of large-scale celestial bodies–obtained from their calculated gravitational interactions—-and the mass measured from the visible atomic matter that they host.
The possible existence of dark matter was first proposed by the Dutch astronomer Jan Oort (1900-1932) as a result of his dedicated effort to understand the orbital velocities of our own Milky Way Galaxy’s constituent stars. In 1933, the Swiss-American astronomer Fritz Zwicky (1898-1974), also proposed the existence of an exotic form of abundant and transparent matter. Zwicky reached this conclusion because he realized that some form of invisible “missing mass” traveled ghost-like through the Cosmos–and that this transparent and invisible exotic material influenced the orbital velocities of constituent galaxies inhabiting distant galaxy clusters. In 1939, strong evidence that the bizarre invisible matter really exists in nature was calculated from galaxy rotation curves by the astrophysicist Horace W. Babcock (1912-2016) of the California Institute of Technology (Caltech) in Pasadena. However, Babcock did not realize that his extremely suggestive observations indicated the presence of dark matter.
Finally, half a century ago, the astronomer Vera Rubin (1928-2016) became the first scientist to offer convincing evidence for the existence of the dark stuff. In the 1960s, Rubin–who had studied Zwicky’s work as a graduate student–proposed her new theory that she based on galactic rotation curves. Soon after Rubin’s study was published, a number of important observations were made by other astronomers that also indicated the existence of this exotic, ghostly form of transparent matter. The later studies were based on observations that used gravitational lensing of background objects by foreground galaxy clusters, the distribution and temperature of hot gas located within individual galaxies and galaxy clusters, and (more recently) the observed pattern of anisotropies seen in the Cosmic Microwave Background (CMB) radiation that formed in the newborn Universe at the time of its birth in the Big Bang. Gravitational lensing is a phenomenon proposed by Albert Einstein in his General Theory of Relativity (1915), when he realized that gravity could distort Spacetime–and, for this reason, have lens-like effects.
The galaxies that perform their fantastic dance throughout the entire visible Universe emerged less than a billion years after the Big Bang. In the very ancient Cosmos the transparent, exotic dark matter snared floating clouds of gas that became the primeval nurseries of the first generation of fiery stars to illuminate what was once a dark and featureless expanse.
At last, the swirling floating gas clouds and the ghostly dark matter met up with one another and performed an ancient waltz throughout the Universe. Gradually, they combined to create the familiar structures that now exist in today’s Cosmos.
The theoretical existence of dark matter is an integral part of recent scenarios describing galaxy birth, evolution, and the formation of cosmic structure. In addition, the real existence of this exotic form of matter is important because it provides an explanation for the anisotropies observed in the CMB–the remnant radiation left over from the Universe’s tumultuous birth. All lines of evidence, so far, indicate that galaxies, galaxy clusters, as well as the entire vast Universe as a whole, contain considerably more matter than can be observed by astronomers using electromagnetic radiation.