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\subsection{Galaxy Evolution: from the Big Bang to present day}  The Big Bang was the birth of our Universe. The early state of the Universe was hot and dense. In this era, matter was coupled to radiation, according to early models of structure formation which assumed adiabatic initial conditions. The Universe expanded and temperature decreased, this occurred approximately 380 000 years after the Big Bang at a redshift of z = 1100 and is depicted in Figure 1. It is also theorized that matter and radiation became decoupled at this same time as well. Redshift (Z) is the increase of apparent wavelength of light coming toward an observer as the result of an object moving away from the observer where $1 + z = \lambda_{\rm obs}/ \lambda_{emit}$. The radiation is a relic of the big-bang that is still observed today; it is called the Cosmic Microwave Background (CMB). The CMB is nearly isotropic blackbody radiation which expanded and cooled and fills the universe and is now $T_{rm 0}=2.725 $T_{0}=2.725  \pm 0.002 K$. \cite{Burke_1997} state that the isotropy of the CMB implies that sections of the universe Universe  that were never in communication with one another have similar properties at the time of observations . observation.  Observations of the CMB communicate that the post- Big-bang post Big Bang  universe is a homogeneous, isotropic expanding or contracting universe, however this is not the reality of the universe. Universe.  It has been further theorized that current structure formation originated from quantum fluctuations. These  fluctuations in give rise to the measured  temperature and density contrast seen  in an isotropic the CMB and large scale structure in a  homogeneous isotropic  universe \cite{Mo_2009}. \cite{Burke_1997} further states state  that the same fluctuations/ irregularities fluctuations  would have been imprinted on the radiation that we now see as the CMB. The quantum fluctuations we resulted from  regions whose density was slightly higher than the mean density of the universe. These regions of higher density attracted surrounding matter through gravity. As a result, slightly-denser regions attracted matter towards them and become even denser. Low-density regions on the other hand become even less dense because matter flows flows  away from them. This amplification of quantum fluctuations is referred to as gravitational instability (Galaxy Formation and Evolution, Houjun Mo, Frank van den Bosch, Simon White). \cite{Mo_2009}.  The initial fluctuations were the epicenters of the newly formed clumps of matter mainly made from hydrogen helium and free floating electrons. ionized plasma.  Thus initial perturbations are the building blocks of atomic nuclei and free floating electrons ionized plasma  which combined via recombination to form neutral atoms, which later formed there is ;however ,  gas clouds, stars, galaxies, andthe  other astronomical astrophysical  structures. There is a significant amount of matter in the universe, which is not directly detectable. Fritz Zwicky observed using the Doppler shift and the virial theorem that the velocity spread within a cluster of galaxies implied that there was more mass than the luminous matter accounted for. Through further experimentation it was later concluded that our universe’s visible matter is baryons were  set ina hallo  matter which does not interact with light, but whose effects are observed through gravitational interactions. This non-luminous matter is called dark matter. The nature of dark matter is still unknown. (Galaxy formation and evolutions, Houjin Mo et al). unknown; however, dark matter is largely expected to be a collisionless particle. \cite{Mo_2009}.  Dark matter influences how galaxies form and even their rotation as it is the frame upon which the visible matter is embedded. The distribution of galaxies also depends on the distribution of dark matter in the universe. (B.F Burke and F Graham-Smith) states \cite{Burke_1997} state  that dark matter may be hot or cold. cold - allowing for different levels of clustering in the Universe.  The Hot Dark Matter (HDM) theory allows for a hierarchy in structure formation. The theory states that large scale structure such as the distribution of galaxies form first due to the presence of hot dark matter. Cold dark matter is theorized to be the seed for galaxy formation. Galaxies are formed from the mergers of stellar material which result in the formation of systems of billions of stars, gas and dust, held together by gravitational attraction. (A Loeb) It has been shown that under appropriate conditions, the process of gravitational collapse within a gaseous cloud, the cloud, or some part of it becomes unstable and begins to collapse if it lacks sufficient gaseous pressure support to balance the force of gravity. The cloud is stable for sufficiently small mass (at a given temperature and radius), but once the Jeans mass (a critical mass) is exceeded, the cloud begins a process of runaway contraction until an external unbalanced force can impedes the collapse. \cite{Loeb_2010}  states that when an object above the Jeans mass collapses, the dark matter forms a halo inside of which the collapsed matter cools, condense to the centre center  of the dark matter halo, and eventually form stars. Dark matter is weakly interacting and is thus unable to cool. Consequently the emergent structure of a galaxy becomes one in which a central core that is occupied by stars and cold gas which is enclosed by dark matter. (A Loeb) \cite{Loeb_2010}  further states that a centrifugal force associated with the rotation of the galaxy’s centre center  prevents the gas from collapsing into the centre center  and forming a black hole. The gas later forms stars and a galaxy is born.(A Loeb)  Radiative cooling, star formation and supernova explosions processes that are also integral to the formation of a galaxy while processes such as accretion of gas, and galaxy mergers, govern the galaxy’s structure. These sets of processes together drive the formation and evolution of galaxies. (D Ceverino at el) galaxies \cite{Ceverino_2009}.  Dark matter halos grow in hierarchy in the sense that larger halos are formed through the merging of smaller predecessors. Figure 2 illustrates the formation of a dark matter halo. In CDM models part From this hierarchical model, it can been seen that present formation and evolution  of galaxies has its roots in  the growth of a massive halo is due to merging with a large number of much smaller halos, where mergers are thought as accretion (to a good approximation). (Mo et al, 2011) Big Bang.