Most galaxies are found in gravitationally-bound groups called “clusters”. Clusters can be rich, with several thousand galaxies, or poor, with only 20 or 30 members. The Local Group, the cluster to which our own Milky Way galaxy belongs, is made up of about 30 galaxies.
Clusters of galaxies are classified by their properties; richness (number of members), shape (spherical, flattened, or irregular),and galactic content (spiral-rich, spiral-poor, or elliptical-rich), for example. Some are strong radio sources, while others emit x-rays. The richest nearby cluster is Virgo, 60 million light years from the Milky Way. It contains about 2500 galaxies, mostly ellipticals
We reside in a small group called the Local Group which is dominated by two giant spiral galaxies, Andromeda and our own Milky Way. In addition to Messier 33, an intermediate mass Sc galaxy, there are 15 ellipticals and 13 irregular galaxies in the cluster, including the Magellanic Clouds, our Galaxy’s satellites, Messier 32 and NGC 205, satellites of Andromeda. The group has a size of about 3 million l.y., and has a total mass of 5 x 1012M
A cloud of dark matter has been detected expanding like a smoke ring from a giant collision between galaxy clusters, a team of astronomers says. If confirmed, the ring could offer new clues to the nature of the mysterious matter.
Dark matter is an enigmatic material that does not emit, absorb or reflect light. It reveals itself only by the way its gravity influences normal matter around it and seems to outweigh the universe’s normal matter by a factor of six.
Now, astronomers have discovered what looks like a ring of dark matter expanding from a cosmic clash involving two massive galaxy clusters. If confirmed, the ring will help astronomers investigate how dark matter behaves when disturbed, perhaps providing hints to its nature.
