Ghostly and invisible, the bizarre dark matter is a mysterious and unidentified form of matter that accounts for approximately 27% of the mass and energy of the observable Universe. Dark matter is thought to be an exotic form of matter that does not interact with light or any other form of electromagnetic radiation, which is why it is transparent and invisible. Although it has not, and cannot, be directly observed, its existence and properties are inferred from its gravitational influence on that which can be seen, such as the motions of visible matter, gravitational lensing, the large-scale structure of the Universe, and its effects on the cosmic microwave background (CMB) radiation–which is the relic radiation of the Big Bang birth of our Universe almost 14 billion years ago. It has been said that the Universe may weirder than we can suppose, and in August 2016 a team of astronomers added still more credibility to that supposition. Their observations reveal that the dark galaxy dubbed Dragonfly 44 is extremely faint for its mass, and it consists almost entirely of the mysterious, ghostly, and invisible dark matter.
The international team of astronomers used the world’s most powerful telescopes to make their important discovery of this massive and very dim galaxy, that is apparently made of 99.99 % of the strange “dark” stuff. Using the W. M. Keck Observatory and the Gemini North telescope–both poised atop the dormant Maunakea volcano in Hawaii–the astronomers were able to detect this bizarre object. The team’s findings are published in the August 25, 2016 issue of The Astrophysical Journal.
Even though the Dragonfly 44 galaxy is relatively nearby, it managed to elude discovery by astronomers for decades because of its extraordinary faintness. It was first discovered, at last, in 2015 when the Dragonfly Telephoto Array stared at a region of the sky in the constellation Coma. Further observations revealed to the astronomers that the galaxy had to have more to it–much more to it–than meets the eye. Dragonfly 44 has so few stars that it would be rapidly ripped to shreds unless there was something there–something invisible–that was holding it together.
The Standard Model of Cosmology indicates that the total mass-energy of the Universe is composed of 4.9% “ordinary” atomic (baryonic) matter, 26.8% dark matter, and 68.3% dark energy. The dark energy, which composes the lion’s share of the Cosmos, is even more mysterious than the dark matter. The currently most-favored explanation for the dark matter proposes that it is a property of space itself, and that it is causing the Universe to accelerate in its expansion. The so-called “ordinary” atomic matter, the runt of the cosmic litter, is really very extraordinary. Atomic matter accounts for literally all of the elements listed in the familiar Periodic Table, and these baryonic elements make our familiar world, and life itself, possible. The iron in your blood, the calcium in your bones, the oxygen that you breathe, the sand beneath your feet, the water that you drink, are all composed of so-called “ordinary” matter. Most of these elements were cooked up in the searing-hot hearts of the Universe’s stars–only hydrogen, helium, and small quantities of lithium and beryllium were produced in the Big Bang. We are such stuff as stars are made of.
Invisible, Ghostly Matter
According to the Standard Model of the formation of cosmic structure, particles of dark matter initially dance together gravitationally to form a crowded place in space, which is termed a dark matter halo. As time passes, these dark halos hoist in–with the irresistible attraction of their powerful gravitational lure–floating clouds of pristine, primordial, primarily hydrogen gas. Hydrogen is the most abundant, as well as the lightest, atomic element in the entire observable Universe. As a result of this ancient beginning, stars and galaxies emerged out of the frigid darkness.
The first galaxies to form in the Universe were likely dark, opaque, and amorphous structures. Shapeless, billowing clouds of ancient gases collected together, and then somersaulted into, the hearts of bizarre halos of the phantom-like, invisible dark matter. Eventually, the first newborn generation of brilliant and sparkling baby stars were born, and their new and roiling flames lit up the first galaxies that served as ancient stellar nurseries.
Dark matter is generally thought to exist because of the very important differences astronomers have observed between the mass of large celestial objects–calculated from their gravitational interactions–and the mass determined from the observable matter that they contain, such as gas, dust, and stars.
The existence of dark matter was first proposed by the Dutch astronomer Jan Oort (1900-1992) in 1932, in order to explain the orbital velocities of stars inhabiting our own Milky Way Galaxy. In 1933, Fritz Zwicky (1898-1974), a Swiss-American astrophysicist at the California Institute of Technology (Caltech), located in Pasadena, California, also proposed the existence of a bizarre form of transparent, and very abundant matter. Zwicky did this in order to explain evidence of “missing mass” lurking ghostlike in the orbital velocities of galaxies dwelling within distant clusters. Strong evidence derived from galactic rotation curves was detected by the Caltech astrophysicist Horace W. Babcock (1912-2003) in 1939, but he did not attribute his suggestive observations to dark matter.
At last, Dr. Vera Rubin (b. 1938), in the 1960s and 1970s, was the first to propose the existence of dark matter based on powerful evidence derived from galaxy rotation curves. Following closely on the heels of Dr. Rubin’s research, a number of important observations were made by other scientists indicating that the phantom-like dark matter was secretly haunting the Universe. These new discoveries were based on observations that included the gravitational lensing of background objects by foreground galaxy clusters such as the Bullet Cluster, the distribution and temperature of extremely hot gas within galaxies and galactic clusters and–more recently–the pattern of anisotropies seen in the CMB radiation left as a tattle-tale relic of the Big Bang. The anisotropies seen in the CMB result from temperature variations in the neonatal Cosmos. Gravitational lensing is a phenomenon proposed by Albert Einstein in his Theory of General Relativity (1915) when he determined that gravity could warp Spacetime and, as a result, bend and distort the path light takes through the Universe–thus producing lens-like effects.
Galaxies ignited less than a billion years after the Big Bang. In the ancient Universe, the mysterious and invisible dark matter grabbed at clouds of mostly hydrogen gas, and pulled them in. These pristine gas clouds were destined to become the ancient cradles of the first stars, lighting up what was then a swath of featureless, merciless blackness with their newborn stellar flames.
Gradually, the swirling, tumbling clouds of primordial, pristine gas and the ghostly, non-atomic dark matter, danced around throughout the ancient Universe. Eventually, they mixed themselves up together to form the distinct structures that we observe today.
The dark matter hypothesis plays a starring role in today’s modeling of cosmic structure formation and galactic birth and evolution, as well as on explanations of the anisotropies seen in the CMB. All of the lines of evidence gathered so far indicate that galaxies, galaxy clusters, and the Universe as a whole harbor much more matter than that which can be seen via electromagnetic signals.
The most widely accepted current theory proposing the identity of dark matter suggests that it is made up of weakly interacting massive particles (WIMPS) that dance around together only through the force of gravity and (to a smaller extent) the weak nuclear force that accounts for certain forms of radioactive decay (beta decay).
Even though the existence of the ghostly dark matter is generally accepted by most astronomers, a minority of astronomers suggest various modifications of the standard laws of General Relativity. These alternative models attempt to explain observations without invoking additional, phantom-like matter.
The Darkest Galaxy
In order to calculate the amount of dark matter contained in Dragonfly 44, the international team of astronomers used the DEIMOS instrument installed on Keck II to measure the velocities of stars for 33.5 hours over a period of six nights so they could determine the galaxy’s mass. The team then went on to use the Gemini Multi-Object Spectrograph (GMOS) on the 8-meter Gemini North Telescope on Maunakea in Hawaii to show a halo of spherical clusters of stars surrounding Dragonfly 44’s core, similar to the halo that encircles our own Milky Way Galaxy.
Motions of the stars tell you how much matter there is,” Dr. Pierter van Dokkum noted in an August 25, 2016 Keck Observatory Press Release. “They don’t care what form the matter is, they just tell you that it’s there. In the Dragonfly galaxy stars move very fast. So there was a huge discrepancy: using Keck Observatory, we found many times more mass indicated by the motions of the stars, than there is mass in the stars themselves,” he added. Dr. van Dokkum is of Yale University in New Haven, Connecticut.
The mass of Dragonfly 44 is estimated to be approximately a trillion times the mass of our Sun. This is about the same mass that our own large starlit Milky Way Galaxy contains. However, only one hundredth of one percent of that is in the form of stars and “ordinary” atomic matter; the other 99.99 percent is in the form of the ghostly, exotic, transparent dark matter. Our Galaxy has more than a hundred times more stars than Dragonfly 44.
Discovering a galaxy with about the same mass as our Milky Way that is amost completely dark was a surprise. “We have no idea how galaxies like Dragonfly 44 could have formed. The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue. But at the moment we’re just guessing,” Dr. Roberto Abraham explained in the August 25, 2016 Keck Observatory Press Release. Dr. Abraham is of the University of Toronto in Canada.
“This has big implications for the study of dark matter. It helps to have objects that are almost entirely made of dark matter so we don’t get confused by stars and all the other things that galaxies have. The only such galaxies we had to study before were tiny. This finding opens up a whole new class of massive objects that we can study,” Dr. van Dokkum explained in the August 25, 2016 Keck Observatory Press Release.
Dr. van Dokkum continued to note that “Ultimately what we really want to learn is what dark matter is. The race is on to find massive dark galaxies that are even closer to us than Dragonfly 44, so we can look for feeble signals that may reveal a dark matter particle.”