Cosmology, the universe consists of ordinary matter

Cosmology, one of the fundamental areas of astronomy, is to study the universe as a whole. The studies on spiral and elliptical nebula in 1920s (Hubble, 1926) are commonly marked as the start of modern cosmology, which first proved that these nebula are essentially galaxies outside the Milky Way. Since then, observations have clearly shown that galaxies gather to form different kinds of cosmic structures, such as groups, clusters, superclusters and galaxy filaments, referring to the web-like distribution of the galaxies and the largest known structures of the cosmos (Bharadwaj, Bhavsar, & Sheth, 2004). How to understand the formation and evolution of these structures has been the research hotspot for decades, and several theories have been proposed. Among them, the cold dark matter (CDM) model (George, Faber, Primack, & Rees, 1984) has been proved to be the most successful theory, especially in explaining the large-scale structures of the universe. The CDM model is based on two hypotheses: the dominance of dark matter (DM) in the evolution of any structure larger than galactic cores, and the negligible thermal velocity of DM in the early universe, with respect to the speed of cosmic expansion.The former hypothesis is developed from the fact that DM constitutes the majority of the cosmic mass. In modern cosmology, the total mass of the universe consists of ordinary matter and DM. The former is the known matter that can be directly observed, such as planets, stars, galaxies and gas. The latter refers to a special kind of matter surrounding the galaxies, whose existence can be deduced through gravitational effects, but has not been directly observed for it does not interact with the electromagnetic wave. According to Ade et al. (2014), DM composes 84.2% of total mass, which indicates the gravitational dominance of DM in the cosmos. If the cosmic structure formed under the gravitational effects, DM is the key factor in this process.The latter hypothesis, suggesting a hierarchical sequence of structure formation, is developed from previous studies. According to George et al.(1984), two main models about DM had been explored until 1984: hot DM (neutrinos ~30 eV) and warm DM (particles ~1 keV). However, both have serious problems being inconsistent with observations because of the excessively high thermal velocity of DM, which results in a top-down sequence of structure formation, which means large structures like superclusters form before galaxies. However, observation shows that galaxy formation occurred before redshift z=3, and supercluster collapsed after redshift z<2. In CDM theory, structure formation follows a bottom-up sequence, which is consistent with observations.Compared with other models, the CDM theory has been the best cosmological model till now. Its advantages are not restricted to the bottom-up sequence, as discussed previously. Another attractive feature is that it provides a calculable method to analyse the mass density in the early universe, which is the key period of forming the galaxies and clusters (George, Faber, Primack, & Rees, 1984). With the assumption that the initial fluctuations of DM are adiabatic in the early universe, the fluctuation of mass density, a key factor in structure formation, can be calculated by characterizing the density perturbation. An additional limitation on the density perturbation can be obtained through the large-scale variation of the microwave background. According to George et al.(1984), calculation shows that mass range of galaxies is  108M??1012M? and the slope of fluctuation spectrum is -2, which both provide a good fit to the observation, such as Tully-Fisher and Faber-Jackson laws. With the use of N-body simulation, this advantage has been further developed to understand the substructures of the universe more precisely (Ghigna et al.,1998). In addition, more reasonable candidates for dark matter are allowed in CDM theory than in others. Considering DM is a phenomenological theory due to the limited knowledge about DM, the number of allowed candidates is crucial. As a result, CDM theory is the most suitable explanation for the formation of cosmic structures.Since 1984, a number of further studies on the CDM paradigm have shown its demonstrable success in predicting galaxy clustering. For example, the N-body simulation was first applied to the CDM models by Ghigna et al. (1998), and provided high resolution for the studies on substructure halos. Dark matter halos originate from the initial fluctuations of DM in the early universe, and provide sites for galaxies formation. In the CDM models, the structure formed hierarchically, which means the DM halos merge to form larger ones gradually. These substructures of DM halos had not been studied since 1998 due to the poor resolution of the simulation. Another improvement on the CDM model is the study on high-redshift intergalactic voids (Viel, Colberg, & Kim, 2008). Intergalactic voids refer to the empty space between galaxy filaments, the web-like distribution of the galaxies. These voids are defined as 'the connected regions in the flux distribution above the mean-flux level' (Viel, Colberg, & Kim, 2008). The flux is calculated with Lyman? forest in the high-resolution spectra of quasars, the galaxies with very high luminosity. The Lyman? forest is a series of absorption lines in the spectra caused by the neutral hydrogen, which is common in high-redshift galaxies. These lines are used as the tracer of DM. Calculation shows that the void distribution perfectly agrees with the prediction of the CDM theory. To sum up, the CDM theory has been tested and studied for decades and remains the most successful theory in cosmology.However,