Every year, as part of its educational and research mission, the Space Telescope and Science Institute in Baltimore, Maryland holds a science symposium that focuses one of the big questions of astronomy, inviting over a hundred scientists to come and give their individual perspectives on the state of the field.

This year’s symposium ended yesterday, and the subject was the mysteries of dark matter. Though I wasn’t able to attend the symposium itself, they held a workshop for journalists yesterday, which I did attend. (You can watch the webcast here.)

So, what is dark matter?

First of all, it isn’t dark energy. Dark energy is that mysterious unknown phenomenon that is causing — on vast scales of many billions of light years — the expansion of the universe to accelerate rather than decelerate. It has nothing to do with the question of dark matter.

Second, no one knows. All that scientists do know is that objects in the outer regions of galaxies as well as the galaxies themselves don’t move at the speeds and directions expected if their known mass and gravity were the only forces influencing them. In order to successfully plot their orbits and motions, astronomers have to add a gigantic halo of extra mass, which they have dubbed “dark matter” because it is unseen, undetected, and completely invisible.

More significantly, the amount of dark matter they need to add ends up comprising the majority of the mass in the cluster. For example, the mass of a typical galaxy cluster, made up of many thousands of galaxies, is about 2% stars, 13% gas, and 85% dark matter. As Megan Donahue of Michigan State University noted at the workshop, “The gas outweighs the stars by factor of 7, while the dark matter outweighs the gas by 7 again.”

Now, it is possible that, on the scales of galaxies and galaxy clusters, the phenomenon astronomers are seeing might be caused by a variation in the strength of gravity itself, rather than this unseen dark matter. Most astronomers however discount this possibility because the theoretical predictions of this modified gravity haven’t matched their observations of galaxy clusters.

Finding out what dark matter is, however, is going to be a difficult job. What makes this hunt so fascinating to me is how it has brought together the work of astronomers, who look at very large objects very far away, with the work of physicists, who look at very very tiny objects very close by.

You see, the two fields have for decades trying to find the same thing, and didn’t at first know it. The theories of physicists had predicted the existence of certain subatomic particles. To prove these theories, they needed to either detect or observe these particles. What they didn’t realize initially was that, ironically, the properties of these theorized subatomic particles just happened to fit the predictions of astronomers for their dark matter.

So, here’s how the research in this field is now playing out. Physicists are trying to discover the dark matter itself, either by producing it in accelerators like the Large Hadron Collider in CERN, Switzerland, or by detecting it in a variety of other detectors, usually built in deeply buried mines or caves in scattered places throughout the world.

Astronomers, meanwhile, either do observations of the largest objects in the universe to see how dark matter affects them, or create simulations that reproduce the long term behavior of millions of galaxies and the dark matter surrounding them. This research in turn acts to constrain the theories of the physicists, which then helps to tell them where to look to either detect or produce the particles.

Magically, the sciences of astronomy and physics are being fused together, the very last thing anyone would have predicted fifty years ago. “It is an extraordinary time,” Mario Livio of STScI noted yesterday at the workshop.