The Invisible Universe
What’s invisible to us is also crucial to our own well-being.
~Jeannette Winterson
 
It is everywhere around us, but we can’t see, smell, taste, touch or hear it, but it is there. How can we know if we can’t sense it? The only way that we know that dark matter even exists is through its gravitational interaction with matter; matter is defined as a substance, solid, liquid, gas or plasma that has mass. 

It is estimated that matter is the oddity in our universe and that dark matter is the norm. Up to 25% of the total matter in the universe is considered to be dark matter, whereas the matter that makes up you and me only accounts for five percent. The rest is made up of energy. 

What exactly is this invisible dark matter that has so much of an impact on our everyday lives?
 
Dark matter was first proposed by Swiss astronomer Fritz Zwicky in 1933 when he was studying the galaxies in the Coma Cluster while working at the California Institute of Technology. He was measuring how the galaxies were moving near the edge of the cluster; however, his calculations showed that to explain the movements, the mass of the cluster would have to be four hundred times greater than observed. Zwicky proposed the idea that there was dark matter to explain the movements. American astronomer Sinclair Smith, in 1937, who was observing the Virgo cluster, found a similar mass discrepancy to explain the movements of the galaxies.

In spite of the observations, the idea of dark matter was treated with skepticism. Then in 1939, American astronomer, Horace Babcock was looking at the rotation of stars in our neighbour galaxy, Andromeda, for his doctoral thesis. In order to explain the movements, Babcock found a discrepancy in the movements of the stars and the masses observed. Instead of looking to dark matter as a resolution, he attributed the discrepancy to absorption of light or modified dynamics of the outer portions of the spiral galaxy. In 1940, pioneering Dutch radio astronomer Jan Oort found the same results when he observed the motions of another galaxy. It was in 1959 that Franz Kahn and Lo Woltjer looked at the motions of the local group of galaxies and concluded that most of the mass was invisible.
 
In 1970, astronomer Vera Rubin and W. Kent Ford Jr. of Carnegie Institution’s department of terrestrial magnetism looked at the motion of stars in Andromeda. Their conclusions showed that there were large amounts of unseen matter in the outskirts of the galaxy. Subsequent observations of other galaxies came up with similar conclusions. The earlier work of Zwicky and Smith was resurrected and dark matter made it onto the scientific world stage. 

Several theories have been proposed to accommodate the apparent discrepancies observed in the cosmos, but none have gained a good foothold in accepted scientific circles.
 
The discovery of dark matter is actually described in a fictional mystery novel. Mystery author, Alex Brett, in his Cold Dark Matter, looked at the discovery of dark matter and created a murder mystery around it.
 
The nature of this dark matter remains elusive but studies in 2006 of twelve nearby dwarf galaxies gave us some inkling of its properties. Particles making up dark matter move at about a speed of nine kilometres per second resulting in temperatures of ten thousand degrees Absolute--higher than expected. Conclusions from this suggest that dark matter is neither hot nor cold, but warm.
 
S. Ahmed put an interesting spin on the nature of dark matter in his interesting debut novel Dark Matter, in which dark matter was actually just ordinary matter that was hidden from our view so that aliens can remain hidden from view.

In order to explain the elements in the universe today, dark matter must be largely non-baryonic; baryons are the large elementary particles that make up ordinary matter, protons and neutrons. This would be in keeping with the predictions of Big Bang nucleosynthesis that formed our universe. 

One early theory for dark matter particles was the neutrino, first postulated by Austrian physicist Wolfgang Pauli as early as 1930. Neutrinos are electrically neutral weakly interacting subatomic particles but are unaffected by electromagnetic forces. Every second, billions of these elusive particles are passing through the Earth undetected. Right now as you are reading this article, neutrinos are passing right through you. 

Due to the fact that neutrinos have no mass, it is unlikely that they are good candidates for dark matter.
 
A more likely form of non-baryonic dark matter is known as WIMPs which is an abbreviation for Weakly Interacting Massive Particles. They are hypothetical heavy particles that rarely interact with other forms of matter. Examples of the particles, both undetected, are known as axions and neutralinos.
 
Axions were first proposed in the early 1970’s to explain a conundrum in quantum physics. Neutralinos, also hypothetical, were proposed by particle physics to explain the supersymmetry relating bosons to fermions which predicts the existence of undetected partners for each particle known to physics; some such partners can be considered to be of a neutral charge, in other words, a neutralino.

Another form of dark matter, first postulated by astrophysicist Kim Greist in 1991, are Massive Compact Halo Objects for MACHOs--chosen to deliberately contrast with WIMPs. Greist's hypothesis states that there are large objects made up of baryonic material that are surrounded by dark matter halos such as brown dwarfs, neutron stars, black holes and even planets. There is no way to see MACHOs directly but we may be able to detect them via a method called gravitational lensing, which was first proposed by Einstein in his general theory of relativity. It works on the principle that any concentration of matter can act as a lens and bend and hence focus light rays from sources behind at a much greater distance. At least one science fiction tale utilizes gravitational lensing as a plot device. NASA scientist, Geoffrey Landis in his short story "Impact Parameter" describes the discovery of a gravitational lens in space. In this case, that lens turns out to be a wormhole that is used by an alien civilization to visit Earth.

Dark matter has also been classified into hot and cold varieties. Light stuff that moves at high velocities is called hot dark matter and the prime candidate is neutrinos; they are no longer considered ideal candidates for dark matter. Cold dark matter on the other hand, is probably made up of WIMPs which move much more slowly. 

Many cosmologists favour the cold dark matter model. However, recent research has shown that dwarf galaxies in the Milky Way’s halo, which is believed to be made up of dark matter, cannot form if the “missing matter” is cold dark matter. Conversely, the concept of hot dark matter fails to explain how the galaxies formed from the Big Bang. New theory, however, looks at the possibility of a compromise in that there could be warm dark matter which may have formed in the minutes after the initial Big Bang rather than the first millionth of a second as indicated by the cold dark matter model.
 
In recent research of twelve dwarf galaxies around our own carried out at the Institute of Astronomy in Cambridge, astrophysicists found that warm dark matter seems a likely candidate. They found that not only do the galaxies contain four hundred times as much dark matter as regular matter, they found that it had a warm temperature as compared with models using hot or cold dark matter. 

Another intriguing prediction of the Big Bang is that dark galaxies made entirely of dark matter should be common. In 2005, British astronomers found the first such galaxy in VirgoH121, 50 thousand light years from the Virgo cluster.  In 2007, Johns Hopkins University and Space Telescope Science Institute scientists reported a vast dark ring in a galaxy cluster that lays five billion light years away.

There are two ways in which dark matter particles are found, one via direct detection and the other by indirect methods. Direct detection instruments containing media to trap the elusive particles are found in deep underground laboratories usually set up in abandoned mines. One of the deepest such instruments was set up in 1992 in an abandoned nickel mine two kilometres underground in Sudbury, Ontario. A recent experiment uses DNA as the method of detection of dark matter particles. A single layer of DNA molecules is placed beneath a thin gold foil. Once a particle of dark matter strikes the nucleus of a gold atom, it will, in turn, rupture the DNA below. Since the sequence of DNA molecules is well-known, scientists can determine the path of the dark matter molecules.  

With a new form of matter being now accepted as scientific fact, is it possible that there are life forms out there that are made up of dark matter? Perhaps there is a dark matter organism sitting right next to you. Maybe that presence in the room is actually a dark matter being. 

Who knows? 

Several science fiction authors have looked at dark matter aliens as being a possibility.

Stephen Baxter, in his Xeelee Sequence of novels, creates a dark matter being called photino birds. Photino birds live in the gravity wells of stars. In order to survive, the birds age stars to a point that they do not go supernova. Since supernovae are essential to the creation of most of the elements of our universe, photino birds are creating a universe which is not suitable to baryonic life forms.

In his novel Starplex, Robert Sawyer describes a number of complex ideas including life forms made of dark matter.

Dark matter is an intriguing concept that started out as a bizarre hypothesis largely ignored by the scientific community when first proposed in the 1930’s. It is sobering and exciting to see that the persistence of early pioneering thinkers has made a hypothetical concept accepted today as scientific fact. 

Who knows what other hypothetical visions of scientists and science fiction authors will move from idea into an accepted reality?
 

Further Reading:
 
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Bertone, G. and Merritt, D. 2005. Dark Matter Dynamics and Indirect Detection. Modern Physics Letters A. 20(14):1021-1036.
 
Bertone, G. et al. 2005. Particle dark matter: Evidence, candidates and constraints. Physics Reports. 405(5-6):279-390.
 
Bertone, G. 2010. The moment of truth for WIMP dark matter. Nature. 468:389-393.
 
Bertone, G. 2013. Particle Dark Matter: Observations, Models and Searches. Cambridge University Press.
 
Bottino, A. 2003. Non-baryonic dark matter. Nuclear Physics B-Proceedings Supplements. 114:27-37.
 
Brownstein, J. and Moffat, J. 2007. The Bullet Cluster 1E0657-558 evidence shows modified gravity in the absence of dark matter. Monthly Notices of the Royal Astronomical Society. 382(1):29-47.
 
Cline, David. 2003. The Search for Dark Matter. Scientific American.
 
Clowe, D. et al. 2006. A direct empirical proof of the existence of dark matter. The Astrophysical Journal. 648:L109-L113.
 
Cohen, Nathan. 1989. Gravity’s Lens: Views of the New Cosmology. Wiley and Sons.
 
Davis, M. et al. 1985. The evolution of large-scale structure in a universe dominated by cold dark matter. Astrophysical Journal. 292:371-394.
 
Dekel, A. and Silk, J. 1986. The origin of dwarf galaxies, cold dark matter, and biases galaxy formation. Astrophysical Journal. 303:39-55/
 
Dekel, A. et al. 2005. Lost and found dark matter in elliptical galaxies. Nature. 437:707-710.
 
Drukier, A. et al. 1986. Detecting Cold Dark Matter Candidates. Physical Review D. 33(12):3495-3508.
 
Faber, S. and Jackson, R. 1976. Velocity dispersions and mass-to-light ratios for elliptical galaxies. Astrophysical Journal. 204:668-683.
 
Feng, J. 2010. Dark Matter Candidates from Particle Physics and Methods of Detection. Annual Review of Astronomy and Astrophysics. 48:495-545.
 
Feng, J. et al. 2000. Neutralino dark matter in focus point supersymmetry. Physics Letters B. 482(4):388-399.
 
Fornengo, N. 2008. Status and perspectives of indirect and direct dark matter searches. Advances in Space Research. 41(12):2010-2018.
 
Freeman, K. and McNamara, G. 2006. In Search of Dark Matter. Birkhauser.
 
Freese, K. 1986. Can Scalar Neutrinos or Massive Dirac Neutrinos be the Missing Mass? Physics Letters B. 167(3):295-300.
 
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Graff, D. and Freese, K. 1996. Analysis of a Hubble Space Telescope Search for red Dwarfs: Limits on Baryonic Matter in the Galactic Halo. The Astrophysical Journal Letters. 456:L49-L53.
 
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Massey, R. et al. 2007. Dark matter maps reveal cosmic scaffolding. Nature. 445:286-290.
 
Minchin, R. et al. 2005. A Dark Hydrogen Cloud in the Virgo Cluster. The Astrophysical Journal Letters. 622:L21-L24.
 
Moore, B. et al. 1998. Resolving the Structure of Dark Matter Halos. The Astrophysical Journal. 499:L5-L8.
 
Moore, B. et al. 1999. Dark Matter Substructure within Galactic Halos. The Astrophysical Journal Letters. 524(1):L19-L22.
 
Ostriker, J. and Steinhardt, P. 2003. New Light on Dark Matter. Science. 300(5627):1909-1913.
 
Percival, W. et al. 2007. Measuring the Baryon Acoustic Oscillation scale using the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey. Monthly Notices of the Royal Astronomical Society. 381(3):1053-1066.
 
Rubin, Vera. 1999. Bright Galaxies, Dark Matters. Springer.

Rubin, V. and Ford, W. 1970. Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions. The Astrophysical Journal. 159:379-404.

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