What’s the (dark) matter?

NASA, ESA, M. J. Jee and H. Ford et al. (Johns Hopkins Univ.)

What sort of stuff makes up our universe? The question is almost a throwback to the ancient Greeks, who first coined the idea that everything is composed of tiny units called atoms, or the later erroneous view that an “ether” fills all of space.

And here we sit, asking the question anew. On March midnights, a vast emptyish zone fills the southern sky between the constellations of Leo and Virgo. The identity crisis manifests in this area, found by following the Big Dipper’s arc in an enormous looping curve toward the blue star Spica. This region is home to the nearest large cluster of galaxies, the Virgo group.

Virgo galaxies sit about 60 million light-years away, which is why there’s no trace of them to the naked eye. Thousands of cities of suns lurk here, and show themselves through backyard telescopes as smudgy blobs peppering the Leo/Virgo boundary. Each has billions of stars. It’s the Times Square, the Main Street of our part of the universe.

The problem is that each galaxy moves so fast that it should easily escape the gravitational clutches of the group as a whole. The galaxy cluster should have dissipated long ago, like a crowd dispersed by the police. It shouldn’t be there at all.

While we’re at it, neither should our own little knot of 30 galaxies that lies at the distant fringes of Virgo’s influence. Nor should any of the other more distant galaxy clusters. Either gravity is oddly weak at great distances, or some unseen glue is holding all these galaxies together. This is the “dark matter” that everyone’s searching for.

To supply this much gravitational attraction, there’s got to be six times more stuff in these galaxies than meets the eye. They can’t be stars, because we’d see stars. They can’t be dusty nebulae, because they’d block the light of the stars. They can’t be black holes, because that would influence motion. The dark area of Virgo, so initially uninteresting this month, proves a metaphor for one of the greatest celestial mysteries: If 85 percent of the cosmos is made of unknown stuff, then we (Sun, Earth and its tormented inhabitants) are an odd, atypical cosmic item – a minority substance in the cosmic census.

Everyone has a different guess. Maybe it will really come down to some fundamentally strange aspect of gravity. But recent evidence indicates that some types of subatomic neutrinos may have a bit of mass. Even if so, there wouldn’t be enough of them to add up to all the missing gravity. Maybe there are a nonillion baseballs out there, or comet nuclei. Every idea has problems. Many can supply some of the dark matter, but not all. It has to be something we cannot see, yet which exerts a gravitational pull.

If the dark matter is an entirely new form of material, unlike the baryonic matter that comprises our bodies and our planet, then we have been demoted once again, big-time. Long ago we were displaced from our assumed position at the center of the universe, then nudged from the center of our galaxy. Now it may be that we are made of material that is not even representative of most of the universe.

If you’ve been feeling odd, maybe that’s the reason.

Want to know more? To read Bob’s previous columns, visit our Almanac Weekly website at HudsonValleyOne.com. Check out Bob’s podcast, Astounding Universe, co-hosted by Pulse of the Planet’s Jim Metzner.