Most galaxies occupy groups or clusters with from 10 to hundreds of members. Each cluster is held together by the gravity from each galaxy. The more mass, the higher the velocities of the members, and this fact can be used to test for the presence of unseen matter.
Early measurements indicated that galaxies in clusters were moving too fast for the amount of matter estiamted from counting the galaxies. This high motion was also detected in other kinematic studies, such as binary galaxies, galaxy rotation and large scale motion of superclusters. This is known as the dark matter problem.
Up to 95% of the mass in clusters is not seen, or dark. Since the physics of the motions of galaxies is so basic, there is no escaping the conclusion that a majority of the matter in the Universe has not been identified, and that the matter around us is special. The question that remains is whether dark matter is baryonic (normal) or a new substance.
From comparing the mass estimates to the observed amount of light from galaxies, and from the abundance of light elements, there is a problem with the amount of mass in the Universe that is in normal matter or baryons. The fraction of light elements indicates that the density of the Universe in baryons is only 2 to 4% what we measure as the observed density. The rest of the mass appears to be 'missing,' meaning unobserved or dark.
Exactly how much of the Universe is in the form of dark matter is a mystery and difficult to determine, obviously because its not visible. It has to be inferred by its gravitational effects on the luminous matter in the Universe (stars and gas) and is usually expressed as the mass-to-luminosity ratio (M/L). A high 'M/L' indicates lots of dark matter, a low M/L indicates that most of the matter is in the form of baryonic matter, stars and stellar remnants plus gas.
A important point to the study of dark matter is how it is distributed. If it is distributed like the luminous matter in the Universe, that most of it is in galaxies. However, studies of 'M/L' for a range of scales shows that dark matter becomes more dominate on larger scales.
Most importantly, on very large scales of 100 Mpc's (Mpc = megaparsec, one million parsecs and kpc = 1000 parsecs) the amount of dark matter inferred is near the value needed to close the Universe. Thus, it is for two reasons that the dark matter problem is important, one to determine what is the nature of dark matter, is it a new form of undiscovered matter? The second is to determine if the amount of dark matter is sufficient to close the Universe.
It is not too surprising to find that at least some of the matter in the Universe is dark since it requires energy to observe an object, and most of space is cold and low in energy. Can dark matter be some form of normal matter that is cold and does not radiate any energy? For example, dead stars?
Once a star has used up its hydrogen fuel, it usually ends its life as a white dwarf star, slowly cooling to become a black dwarf. However, the timescale to cool to a black dwarf is thousands of times longer than the age of the Universe. High mass stars will explode and their cores will form neutron stars or black holes. However, this is rare and we would need 90% of all stars to go supernova to explain all of the dark matter.
Another avenue of thought is to consider low mass objects. Stars that are very low in mass fail to produce their own light by thermonuclear fusion. Thus, many, many brown dwarf stars could make up the dark matter population. Or, even smaller, numerous Jupiter-sized planets, or even plain rocks, would be completely dark outside the illumination of a star. The problem here is that to make-up the mass of all the dark matter requires huge numbers of brown dwarfs, and even more Jupiter's or rocks. We do not find many of these objects nearby, so to presume they exist in the dark matter halos is unsupported.
An alternative idea is to consider forms of dark matter not composed of quarks or leptons, rather made from some exotic material. If the neutrino has mass, then it would make a good dark matter candidate since it interacts weakly with matter and, therefore, is very hard to detect. However, neutrinos formed in the early Universe would also have mass, and that mass would have a predictable effect on the cluster of galaxies, which is not seen.
Another suggestion is that some new particle exists similar to the neutrino, but more massive and, therefore, more rare. This Weakly Interacting Massive Particle (WIMP) would escape detection in our modern particle accelerators, but no other evidence of its existence has been found.
The more bizarre proposed solutions to the dark matter problem require the use of little understood relics or defects from the early Universe. One school of thought believes that topological defects may have appeared during the phase transition at the end of the GUT era. These defects would have had a string-like form and, thus, are called cosmic strings. Cosmic strings would contain the trapped remnants of the earlier dense phase of the Universe. Being high density, they would also be high in mass but are only detectable by their gravitational radiation.
Lastly, the dark matter problem may be an illusion. Rather than missing matter, gravity may operate differently on scales the size of galaxies. This would cause us to overestimate the amount of mass, when it is the weaker gravity to blame. There is no evidence of modified gravity in our laboratory experiments to date.
The current observations and estimates of dark matter is that 20% of dark matter is probably in the form of massive neutrinos, even though that mass is uncertain. The another 5% to 10% is in the form of stellar remnants and low mass, brown dwarfs. The rest of dark matter is called CDM (cold dark matter) of unknown origin, but probably cold and heavy. The combination of all these mixtures only makes 20 to 30% the amount mass necessary to close the Universe. Thus, the Universe appears to be open, i.e. 'ΩM' is 0.3.
With the convergence of our measurement of Hubble's constant and 'ΩM,' the end appeared in site for the determination of the geometry and age of our Universe. However, all was throw into turmoil recently with the discovery of dark energy. Dark energy is implied by the fact that the Universe appears to be accelerating, rather than decelerating, as discovered by observations of distant supernovae.
The most direct cosmological observation you can make is to find some standard candle, an object with a known luminosity, and follow its change in apparent luminosity with distance (like watching the headlights of a distant car). The problem is most objects, like galaxies, change in brightness from the past till now. One object which is constant is the brightness of a supernova, but until recently the technology to capture them at high distances was not avaliable. Below is the results of the high redshift supernova project, a unique combination of space and ground-based telescope work.
This new observation implies that something else is missing from our understanding of the dynamics of the Universe, in math terms this means that additional cosmological constant in Friedmann's equation, 'Λ.' The implication here is that there is some sort of pressure in the fabric of the Universe that is pushing the expansion faster. A pressure is usually associated with some sort of energy, we have named it 'dark energy'. Like dark matter, we do not know its origin or characteristics.
With a cosmological constant, there are many possible types of Universes, almost any kind of massive or light, open or closed curvature, open or closed history is possible. Also, with high 'Λ's,' the Universe could race away.
Fortunately, observations, such as the 'SN' data and measurements of the cosmic microwave background constrain the possible values for the density of matter and the cosmological constant. The following diagram displays the results and error ellipses for these three observations. The density of matter is constrained by cluster observations to be around 0.3. The flatness of the 'CMB' forces a k=0 Universe, as expected by inflationary cosmology. Lastly, the 'SN' data constrains the value of the cosmological constant.
'SN' data gives ΩΛ=0.7 and ΩM=0.3. This results in Ωk=0, or a flat curvature. This is sometimes referred to as the Benchmark Model which gives an age of the Universe of 12.5 billion years.
Is this the end of cosmological work? Unlikely, continued information still flows in on the conditions near the Big Bang. For example, the following is a list of the current models being considered to explain the first moments of the Universe: Quasi-steady state (non-cosmological redshifts), NUT space (microlensing space), Brans-Dicke gravitation (dark energy couples to dark matter and baryons), Godel cosmology (inclusion of shear and rotation), Lyra geometry (2D singularities or domain walls), complex topologies (dodecahedral shaped universe), Cardassian expansion (non-linear Hubble's constant), Quantized everything (particles based on radius of universe), self-creation comsology (no horizons), cosmological synchronization (cosmological factors affect fluctuation processes), backward universe (evolution proceeds to less order).
Physics of the early Universe is at the boundary of astronomy and philosophy since we do not currently have a complete theory that unifies all the fundamental forces of Nature at the moment of Creation. In addition, there is no possibility of linking observation or experimentation of early Universe physics to our theories (i.e. its not possible to `build' another Universe). Our theories are rejected or accepted based on simplicity and aesthetic grounds, plus there power of prediction to later times, rather than an appeal to empirical results. This is a very different way of doing science from previous centuries of research.
Our physics can explain most of the evolution of the Universe after the Planck time (approximately 10-43 seconds after the Big Bang).
However, events before this time are undefined in our current science and, in particular, we have no solid understanding of the origin of the Universe (i.e. what started or 'caused' the Big Bang). At best, we can describe our efforts to date as probing around the 'edges' of our understanding in order to define what we don't understand, much like a blind person would explore the edge of a deep hole, learning its diameter without knowing its depth.
One of the reasons our physics is incomplete during the Planck era is a lack of understanding of the unification of the forces of Nature during this time. At high energies and temperatures, the forces of Nature become symmetric. This means the forces resemble each other and become similar in strength, i.e. they unify. When the forces break from unification (as the Universe expands and cools) interesting things happen.
An example of unification is to consider the interaction of the weak and electromagnetic forces. At low energy, photons and 'W','Z' particles are the force carriers for the electromagnetic and weak forces. The 'W' and 'Z' particles are very massive and, thus, require a lot of energy (E=mc2). At high energies, photons take on similar energies to 'W' and 'Z' particles, and the forces become unified into the electroweak force.
There is the expectation that all the nuclear forces of matter (strong, weak and electromagnetic) unify at extremely high temperatures under a principle known as Grand Unified Theory, an extension of quantum physics using as yet undiscovered relationships between the strong and electroweak forces.
The final unification resolves the relationship between quantum forces and gravity (supergravity).
In the early Universe, the physics to predict the behavior of matter is determined by which forces are unified and the form that they take. The interactions just at the edge of the Planck era are ruled by supergravity, the quantum effects of mini-black holes. After the separation of gravity and nuclear forces, the space-time of the Universe is distinct from matter and radiation.