Dark matter was originally proposed by astronomers to explain why galaxies rotate much faster than can be explained by the amount of visible matter they contain. This mysterious form of matter does not emit or absorb electromagnetic radiation -- hence the name "dark" -- and can only be detected by its gravitational influence on ordinary matter. According to the standard model of cosmology, the universe is thought to contain about 5% ordinary matter, 25% dark matter and 70% dark energy -- the nature of which is unknown.

Galaxy clusters are the largest gravitationally bound systems in the universe and are composed of three major components: visible galaxies, a hot intra-cluster medium, and dark matter. Jee and co-workers employed a "gravitational lensing" technique to map dark matter from two galaxy clusters -- thought to have formed when the universe was much less than half its present age -- in the southern sky using the Advanced Camera for Surveys (ACS) instrument on the Hubble Space Telescope.

Astronomical bodies, such as galaxy clusters, behave as lenses because their gravitational fields bend the light emitted by background galaxies towards Earth as they pass throught the clusters. This means astronomers can observe dark matter despite it being invisible. Using the Hubble data eliminates the problem of atmospheric turbulence, which blurs similar images taken by ground-based based telescopes.

Using mass reconstruction techniques, Jee and colleagues show how "clumps" of dark matter surround the cluster galaxies (see figure). Although these clumps have been seen before, Jee and colleagues were able to image them in much more detail and discovered additional "satellite" clusters within the clusters.

According to the researchers, the results support the theory that dark matter and visible matter should be found in the same places because gravity pulls them both together. They also agree with current theories on how dark matter particles do not collide, unlike ordinary matter particles, but simply pass through each other. "If dark matter particles were collisional, we would observe a much smoother distribution of dark matter without any small-scale clumpy structures," explains Jee.