Galaxies aren't randomly distributed throughout the universe -- they tend to exist in galactic clusters. The galaxies in these clusters are bound together gravitationally and influence one another.
- Rich clusters contain 1,000 or more galaxies. The Virgo supercluster, for example, includes more than 2,500 galaxies and is located about 55 million light years from Earth.
- Poor clusters contain less than 1,000 galaxies. The Milky Way and the Andromeda galaxy (M31) are the major members of the Local Group, which contains 50 galaxies.
When astronomers Margaret Geller and Emilio E. Falco plotted the positions of galaxies and galactic clusters in the universe, it became clear that galactic clusters and superclusters are not randomly distributed. They're actually clumped together in walls (long filaments) interspersed with voids, which gives the universe a cobweb-like structure.
The intergalactic medium -- the space between galaxies and clusters of galaxies -- is not entirely empty. We don't know the exact nature of the intergalactic medium, but it probably contains a relatively small density of gas. Most of the intergalactic medium is cold (about 2 degrees Kelvin), but recent X-ray observations suggest that some areas of it are hot (millions of degrees Kelvin) and rich in metals. One of the active areas of astronomical research today is directed at determining the nature of the intergalactic medium -- it may help us figure out exactly how the universe began and how galaxies form and evolve.
Let's look at one final property concerning galaxies and their distributions. For his measurements of galactic distances, Edwin Hubble studied the spectra of light that galaxies emit. In all cases, he noted that the spectra were Doppler-shifted to the red end of the spectrum. This indicates that the object is moving away from us. Hubble noticed that, no matter where he looked, galaxies were moving away from us. And the farther the galaxy, the faster it was moving away. In 1929, Hubble published a graph of this relationship, which has become known as Hubble's Law.
Mathematically, Hubble's Law states that the velocity of recession (V) is directly proportional to the galactic distance (d). The equation is V = Hd, where H is the Hubble constant, or constant of proportionality. The most current estimate of H is 70 kilometers per second per megaparsec. Hubble's Law is a major piece of evidence that the universe is expanding -- his work formed the basis of the big bang theory of the origin of the universe.
Some galaxies spew gases, emit intense light and have supermassive black holes at their centers. We'll learn about active galaxies next.