A new study of the after-effects of a collision between two clusters of galaxies provides additional evidence for the existence of dark matter. Dark matter is so dark - how dark is it? - it’s so dark that it doesn’t interact with (emit, reflect, scatter, absorb) light at all. We can’t see it, but we know it’s there because the mass of the dark matter does exert a gravitational force on all the good old regular matter that we can see. If you throw a tennis ball you can predict the path it will follow to very high precision because we know the mass of the Earth. For a cooler example, Cassini mission planners are exploring the possibility of flying within 25 km (about 15 miles) of the surface of Saturn’s moon Enceladus in a couple of years. Bear in mind that both Cassini and Enceladus are hurtling around Saturn at several miles per second and that Cassini is about 1 hour and 20 light minutes from Earth meaning there is no remote control steering of this spacecraft. If it were coming too close to Enceladus there is nothing we could do about it. Yet the only concern with this close encounter is the possibility of small grains of ice produced by Enceladus’s cool geysers smashing into some sensitive pieces of Cassini. The navigation team at JPL is fully confident in their ability to have Cassini pass so close to the surface of Enceladus because we understand very precisely how gravity makes things move.
Which brings us back to dark matter. Observers have noticed for decades that the stars and gas around distant galaxies were moving faster than they should based on how much mass could be seen and inferred to exist within the galaxy. The conclusion of this (and many other observations) is that there is additional dark matter within the galaxy whose only effect is on the trajectories of everything else. The new study, led by Doug Clowe at the University of Arizona, analyzed the distribution of matter in the Bullet Cluster of galaxies where a collision between two clusters some 100 million years ago or so separated the gas from the stars. Clowe’s group was able to show that the extra matter was still with the stars in the galaxies and not with the gas. Since cool gas has been an alternative explanation for the dark matter observations, this new observation significantly strengthens the case for dark matter because if the extra mass were gas it would have been stripped away from the galaxies along with the visible gas. Instead it appears to have stayed with the galaxies (based on their stellar motions) just as expected for dark matter.
So what is this stuff, anyway? There are some exotic candidates, but perhaps the most intriguing thing is that the majority of the universe is not ordinary matter or even dark matter, but the cleverly named dark energy, which is the name for the unknown thing that seems to be pushing the universe apart (faster than it would be going from the initial Big Bang and inflationary expansion). If you don’t already feel insignificant enough when you look at the night sky, all that you and the Hubble Space Telescope can see accounts for only 4-5% of the universe. 22% or so is dark matter and the vast majority is dark energy, and as far as understanding either one, we’re pretty much in the dark.