Why do we believe galaxy clusters are rich in dark matter? What is believed to be the large- scale structure of the universe? Solution 1) Dark matter is a mysterious cosmic phenomenon that accounts for 27 percent of all matter and energy. Though dark matter is all around us, we cannot see it or feel it. But scientists can infer the presence of dark matter by looking at how normal matter behaves around it. Galaxy clusters, which consist of thousands of galaxies, are important for exploring dark matter because they reside in a region where such matter is much denser than average. Scientists believe that the heavier a cluster is, the more dark matter it has in its environment. But new research suggests the connection is more complicated than that. A new study suggests that the internal structure of a galaxy cluster is linked to the dark matter environment surrounding it. Hence this property besides the mass of a cluster is shown to be associated with surrounding dark matter. Researchers studied approximately 9,000 galaxy clusters from the Sloan Digital Sky Survey DR8 galaxy catalog, and divided them into two groups by their internal structures: one in which the individual galaxies within clusters were more spread out, and one in which they were closely packed together. The scientists used a technique called gravitational lensing -- looking at how the gravity of clusters bends light from other objects -- to confirm that both groups had similar masses. But when the researchers compared the two groups, they found an important difference in the distribution of galaxy clusters. Normally, galaxy clusters are separated from other clusters by 100 million light-years on average. But for the group of clusters with closely packed galaxies, there were fewer neighboring clusters at this distance than for the sparser clusters. In other words, the surrounding dark-matter environment determines how packed a cluster is with galaxies. 2) The standard model of cosmology indicates that the total mass–energy of the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.Thus, dark matter constitutes 84.5% of total mass, while dark energy plus dark matter constitute 95.1% of total mass–energy content. The great majority of ordinary matter in the universe is also unseen, since visible stars and gas inside galaxies and clusters account for less than 10% of the ordinary matter contribution to the mass-energy density of the universe..

1. Why do we believe galaxy clusters are rich in dark matter? What is believed to be the large-
scale structure of the universe?
Solution
1)
Dark matter is a mysterious cosmic phenomenon that accounts for 27 percent of all matter and
energy. Though dark matter is all around us, we cannot see it or feel it. But scientists can infer
the presence of dark matter by looking at how normal matter behaves around it.
Galaxy clusters, which consist of thousands of galaxies, are important for exploring dark matter
because they reside in a region where such matter is much denser than average. Scientists believe
that the heavier a cluster is, the more dark matter it has in its environment. But new research
suggests the connection is more complicated than that.
A new study suggests that the internal structure of a galaxy cluster is linked to the dark matter
environment surrounding it. Hence this property besides the mass of a cluster is shown to be
associated with surrounding dark matter.
Researchers studied approximately 9,000 galaxy clusters from the Sloan Digital Sky Survey DR8
galaxy catalog, and divided them into two groups by their internal structures: one in which the
individual galaxies within clusters were more spread out, and one in which they were closely
packed together. The scientists used a technique called gravitational lensing -- looking at how the
gravity of clusters bends light from other objects -- to confirm that both groups had similar
masses.
But when the researchers compared the two groups, they found an important difference in the
distribution of galaxy clusters. Normally, galaxy clusters are separated from other clusters by
100 million light-years on average. But for the group of clusters with closely packed galaxies,
there were fewer neighboring clusters at this distance than for the sparser clusters. In other
words, the surrounding dark-matter environment determines how packed a cluster is with
galaxies.
2)
The standard model of cosmology indicates that the total mass–energy of the universe contains
4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.Thus, dark matter constitutes
84.5% of total mass, while dark energy plus dark matter constitute 95.1% of total mass–energy
content. The great majority of ordinary matter in the universe is also unseen, since visible stars
and gas inside galaxies and clusters account for less than 10% of the ordinary matter contribution
to the mass-energy density of the universe.