DARK MATTER

Dark matter is theoretical matter that has been inferred to exist from gravitational effects on visible matter and background radiation, but is undetectable by emitted or scattered electromagnetic radiation. Its existence was predicted to account for discrepancies between measurements of the mass of galaxies,clusters of galaxies and the entire universe made through dynamical and general relativistic means, and measurements based on the mass of the visible "luminous" matter these objects contain: stars and the gas and dust of the interstellar and intergalactic media. It must be understood that the names "dark matter" and "dark energy" serve mainly as expressions of human ignorance, much like the marking of early maps with "terra incognita".

Dark matter was speculated by Fritz Zwicky in 1934 to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters. Subsequently, other observations such as the rotational speeds of galaxies, gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies also indicated the presence of dark matter.

Dark matter accounts for 23% of the mass-energy density of the observable universe, while the ordinary matter accounts for only 4.6% (the remainder is attributed to dark energy). From these figures, dark matter constitutes 80% of the matter in the universe, while ordinary matter makes up only 20%.

Although dark matter was inferred by a number of astronomical observations, the composition of dark matter remains speculative. Early theories of Dark matter concentrated on hidden massive normal objects, such as blackholes, Neutron Stars, faint old white dwarfs, brown dwarfs, as the possible candidates for dark matter, collectively known as MACHOs. Determining the nature & properties of this missing mass is one of the most important problems in modern cosmology and particle physics.

A unique property of all dark matter is that it behaves like and is modeled like a perfect fluid, meaning that it does not have any internal resistance or viscosity. This means that particles of dark matter should not interact with each other than through gravity, i.e. they must move past each other without even bumping or colliding.

Based on what we know, three categories of dark matter candidates have been postulated. The categories namely cold, warm, and hot refer to the speed at which the particles are traveling rather than an actual temperature.

• Cold dark matter – objects that move at classical velocities
• Warm dark matter – particles that move relativistically
• Hot dark matter – particles that move ultra-relativistically

Dark matter plays a central role in modeling of structure formation and galaxy evolution, and has measurable effects on the anisotropies observed in the cosmic microwave background. All these evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation.

Dark Matter literally screams at us ‘There is more to the Universe than meets the eye, or even ultra-high-tech telescopes' because, in the field of Cosmology, with every answer we find, we find that we raise even more baffling questions.

Questions to ponder:

• What is Dark Energy and how is it different from Dark Matter?
• What could be the reason that Dark Matter constitutes 80% of all matter in the Universe? (Obviously, it must have a central role. Otherwise, it’s an awful waste of space, isn’t it?)