In one form or another, we're all used
to how gravity behaves - it alters the paths of massive objects.
If it weren't for gravity, a ball you throw should go in a straight
line forever - the pull of gravity causes its path to bend toward
Einstein's theory of general relativity, the "pull" of
gravity is actually caused by the bending of space. A large mass
(like the Earth) distorts the fabric of spacetime in its vicinity, and
this distortion alters the path of other objects. One can imagine
this by thinking of spacetime as similar to a big rubber sheet - every
object makes a dent in the sheet (bigger objects make deeper dents),
and the paths of marbles rolling on the sheet are affected by the
presence of these dents.
One consequence of Einstein's work is that gravity should even bend the
path of a beam of light (or x-rays, or radio waves, or any other
electromagnetic radiation) in a particular way. This means that a
big, massive object can distort the image of a distant light source in
a manner similar to an ordinary magnifying glass. This effect is
known as gravitational lensing
and it is one of the hottest fields of study in modern cosmology.
A beautiful example is shown in the Hubble Space Telescope picture
for places in which a massive, relatively nearby
cluster of galaxies sits almost exactly between the Earth and a bright,
distant galaxies and quasars. By studying how the images of these
distant objects are distorted by the gravity of the big cluster, we can
calculate how massive the cluster is.
Once we've used this method to estimate how massive the cluster is, we
can compare this answer to the amount of visible matter. By using
the details of the image distortion, we can also get a rough "map" of
the distribution of the dark matter in the galaxy cluster.
Cosmologists have concluded that galaxy clusters (like galaxies
themselves) are immersed within enormous clouds of dark matter that
outweigh the stars by a factor of ~10.
For more details, see pages by Joanne Cohn
and Pete Newbury
Jeff Filippini, UC Berkeley
Cosmology Group (August 2005)