The short answer is no one really knows. Dark matter is a name given to something that, so far, has only been detected by the gravitational force that it exerts on visible matter and light. In fact, that gravitational force is the evidence for dark matter.
A quick look at “Average Orbital Velocity” in Clark Planetarium’s Solar System Fact Sheet will reveal that the farther a planet is from the Sun, the slower it moves. In contrast, stars and gas in the outer regions of galaxies all have roughly the same speed regardless of their distance from the center. How do astronomers measure their speed? Using the Doppler Effect. Below is an example from a nearby galaxy known as M33. The image on the right is a radio telescope image showing the distribution of hydrogen throughout the galaxy (hydrogen atoms give off radio waves with a wavelength of 21 centimeters). Colors in the image show the Doppler shift of the radio waves. Blue shows hydrogen that is moving toward us. Red shows hydrogen that is moving away.
Since Doppler measurements reveal that stars and gas in the outer regions of galaxies all have similar speeds, this implies that mass in a galaxy must increase with increasing distance. But visible matter in most galaxies appears to decrease with increasing distance from the center. So, unless our understanding of the basic laws of physics needs a tweak (as has been proposed by some) most of the mass in galaxies cannot be seen, hence the name dark matter.
A number of possibilities have been suggested for this unseen mass, from dim stars, brown dwarfs and black holes to exotic and not so exotic sub-atomic particles. Recent searches seem to favor sub-atomic particles. However, one of the exciting things about astronomy research into the unknown is that an unexpected discovery or a better observation can revise current thinking.
One of the best evidences for the existence of dark matter comes from the Hubble Space Telescope. Astronomers used gravitational bending of light from faint distant galaxies to map the distribution of mass in what appears to be the aftermath of a collision between two galaxy clusters. The blue color in the Hubble image shows the distribution of matter based on these measurements. In addition to the mass of the cluster in the center, a ring of unseen mass surrounds the galaxies. Computer simulations suggest that such a collision, occurring along Earth’s line of sight could produce this ring of dark matter.
What is dark matter? Does it really exist? Stay tuned.