Physics for the 21st Century logo  Section 2: Initial Evidence
of Dark Matter  Fritz Zwicky, an astronomer at the California
Institute of Technology, stumbled across the gravitational
effects of dark matter in the early 1930s while studying how
galaxies move within the Coma Cluster. The Coma Cluster consists
of approximately 1,000 galaxies spread over about two degrees on
the sky—roughly the size of your thumb held at arm's length, and
four times the size of the Sun and the Moon seen from Earth.
Gravity binds the galaxies together into a cluster, known as a
galaxy cluster. Unlike the gravitationally bound planets in our
solar system, however, the galaxies do not orbit a central heavy
object like the Sun and thus execute more complicated orbits.
To carry out his observations, Zwicky persuaded Caltech to build
an 18-inch Schmidt telescope that could capture large numbers of
galaxies in a single wide-angle photograph. He used the
instrument to make a survey of all the galaxies in the cluster
and used measuremen