And here we sit, asking the question anew. In September, a vast emptyish zone fills the northwestern sky. The identity crisis manifests in this area, because there's so much "nothing" here - it's the direction the Hubble folks chose to stare, when they wanted an image that would peer all the way the edges of the cosmos, without obstructions.
Along the way, Hubble imaged faraway clusters of galaxies, rendered into smudges by the yawning distances. There's one 250 million light years away, there's one two billion light years, its members showing no detail at all. Each of the smudges has billions of stars.
The problem is that each galaxy moves so fast that it should easily escape the gravitational clutches of the group it's in. But it doesn't. The galaxy clusters should have dissipated long ago, like a crowd dispersed by the police. They shouldn't be there at all.
Fritz Zwicky noticed this effect in the nearest cluster of galaxies, our own local group, way back in 1933. He realized that some unseen glue is holding all of them together.
This is the "dark matter." It's not a small matter at all.
To supply this much gravitational attraction, there's got to be six times more stuff in every galaxy than meets the eye. They can't be stars because we'd see stars. They can't be nebulas because they'd influence the light of the stars. They can't be black holes because that would sway local motion. The dark area of space, so initially uninteresting this month, proves a metaphor for the mystery, second only to the bizarre anti-gravity force called dark energy.
If 96 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. Recent evidence indicates that 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 stuff. 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, but 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, and constantly passes through our bodies without effect, the way neutrinos already do, then we have been demoted once again. 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 the universe.
Whatever it is, the very term dark matter provides a metaphor for how far we have yet to go, in grasping the inventory of the cosmos. It is "dark" not just because it neither emits nor reflects light. It is "dark" as in the old term "darkest Africa" - meaning dark to our minds; a place where no light is being shed upon our intellects.