Our universe is a wonderful, yet confounding place.
In the last few decades, we've come to realize that 84.5 percent of the universe's matter is invisible. Given the rather awkward moniker "dark matter," this stuff exists in a state that doesn't interact with "ordinary" matter. Like "dark energy," these things are "dark" because we don't understand what they are.
If there was a lump of dark matter sitting on my desk, I'd have no way of knowing it was there. In fact, the dark matter lump couldn't "sit" on my desk at all. It would fall through the desk and floor and down into Earth's crust, speeding toward the gravitational well at the core of our planet. Or it might zoom off inexplicably into space. Dark matter is so weakly interacting that the lump would pass through all ordinary matter as if it weren't even there.
On small scales, the gravitational effect of dark matter is minuscule, but over cosmological distances, dark matter's presence is most certainly felt — it can be indirectly observed from its gravitational influence on clusters of galaxies and its impact on the spin of galaxies. So we know it's out there, we just can't see it.
However, we don't know what "it" is, though there are theories.
Ordinary matter — aka baryonic matter — interacts via the electromagnetic, gravitational, weak and strong nuclear forces. These forces transfer energy and provide the structure of all matter. Dark matter, on the other hand, is usually viewed as an amorphous cloud of "stuff" that cannot interact via the electromagnetic, weak or strong forces. It is therefore assumed that dark matter is "nonbaryonic." (Although there are some theories that hint at a baryonic source of dark matter — notably MACHOs, or massive compact halo objects. Only the gravitational effects of nonbaryonic matter can be observed when enough of the stuff clumps together.
The leading candidate in the dark matter search is the aptly named WIMP, an acronym for weakly interacting massive particle. As its name suggests, this hypothetical particle doesn't interact with normal matter — it is therefore nonbaryonic.
Established cosmological models predict that dark matter — be it WIMPs or some other form like "axions" — gives our universe structure and is usually oversimplified as the "glue" that holds our universe together. Clumping like an expanding 3-D web, dark matter influences the evolution of galaxies. Like beads on a string, these dark matter clumps gravitationally corralled ordinary matter into clusters of galaxies with vast empty voids in between.
Astronomer Vera Rubin, while observing the spin of galaxies, first noted that most of the matter in galaxies cannot be seen. Only a small percentage is visible as the stars, gas and dust; the rest is held in a large yet invisible dark matter halo. It's as if our visible galaxy of ordinary matter is merely the hubcap of a massive dark matter wheel that extends far beyond the bounds of what we can see.
In research published by Randall and her team in 2013, a more complex view of dark matter was explored and applied to this massive "dark galaxy" that our visible galaxy is embedded in. In this view, our galaxy's dark matter halo isn't composed of just one type of amorphous mass of nonbaryonic matter.
"It seems very odd to assume that all of dark matter is composed of only one type of particle," Randall writes in her op-ed. "... an unbiased scientist shouldn't assume that dark matter isn't as interesting as ordinary matter and necessarily lacks a diversity of matter similar to our own."