What is Dark Matter and what does it do?

By definition, dark matter is any substance that behaves gravitationally like ordinary matter, but that is invisible to present-day telescopes and detectors. Its existence was first invoked a few decades ago, when astronomers made the startling discovery that the material in the outskirts of spiral galaxies is moving much more rapidly than anticipated from the gravitational pull of their gas and stars alone.

Countless comparisons between the amount of visible matter in astronomical objects and their masses inferred from motions under gravity have since been undertaken. Almost all of these experiments reach the same conclusions: nearly 90% of all matter is dark, and dark and luminous matter combined account for less than 1/3 of total energy density of the Universe (the rest stems from vacuum energy, or dark energy). Quite literally, then, there is much more to space than meets the eye!

While the nature of dark matter remains unknown, observations have revealed 2 of its key properties: First, dark matter is "cold": its characteristic speed must be much lower than that of light even in the very early Universe (otherwise, the smallest galaxies in the vicinity of the Milky Way and other larger systems wouldn't form). Second, dark matter is "collisionless": there are very few interactions among dark matter particles or between dark and luminous particles (in other words, dark matter is indeed invisible, or very nearly so).

The cold collisionless dark matter that pervades the Universe is therefore very different from ordinary matter. It doesn't even contain protons and neutrons, the particles that make up almost everything on Earth. Rather, leading dark matter candidates are exotic weakly interacting massive particles (WIMPs) expected from the supersymmetry theory of particle physics; experiments to test this hypothesis are underway.

Because the amount of dark matter in galaxies and clusters of galaxies dwarfs the amount of ordinary matter they contain, it dominates most gravitational interactions on these scales. For example, the gravitational clustering of dark matter is largely responsible for the formation of galaxies and clusters in the first place!

On smaller scales such as in the Solar System and on Earth, however, dark matter has a negligible gravitational effect. That's because the average dark matter density is much lower (a trillion trillion times lower, in fact!) than that of rocks, water and other substances typically found on Earth. So even though there is dark matter in the Solar System, the latter just isn't big enough for dark matter to play an important dynamical role.

So while dark matter reigns supreme throughout most of the Universe, here at home the protons and neutrons run the show!