Shedding light on dark matter

It generally happens at about 3 a.m. on camping trips. Nature calls, I get out of my sleeping bag and trip over some kind of dark matter.

Perhaps you’ve heard of the stuff. It’s all the rage in astronomical circles. Dark matter has the honor of being the in-vogue cosmic mystery of the 1990s. It accounts for 99.99 percent of the mass of the universe and scientists don’t know what it is.

Strictly speaking, dark matter is precisely what its name implies: matter that is dark. It could be just about anything that doesn’t give off electromagnetic radiation: That means visible light, x-rays, gamma rays, heat, radio waves, ultra-violet or anything else. Dark mater is just dark stuff floating around in the vast spaces between stars. So when we look up at the night sky – even with the most powerful, high-tech telescopes – we’re only seeing a fraction of a percent of all the stuff that’s out there.

The natural question that arises at this point is "if we can’t see it and don’t know what it is, how do we know there is so much of it out there?" It turns out that like quarks and bosons and some other exotic of physics and astrophysics, dark matter is predicted to exist based on observations of how the cosmos behaves.

Specifically, Swiss astronomer Fritz Zwicky first stumbled onto dark matter at Caltech in 1933 while studying how a certain cluster of galaxies in the constellation Virgo dance around each other. Zwicky was measuring the amount of light the galaxies were giving off in order to calculate their masses. But when he was done, he ran straight into a surprising paradox. According to Zwicky’s calculations, these galaxies were lightweights that just couldn’t possibly be moving at their measured speeds without flying apart. There just wasn’t enough visible matter in them to account for the gravity needed to hold them all together. Some unseen stuff was adding a lot of gravity to those galaxies and keeping them from blowing to pieces.

It wasn’t until about 15 years ago that astronomers began seriously pondering and expanding on Zwicky’s work. By that time the missing matter was causing all sorts of problems with their models and theories, and dark matter answered a lot of questions. It’s generally accepted today that dark matter exists, but just what it is remains a mystery.

The earthbound sorts of dark matter I have personally encountered include my dog, Sancho. On those dark desert nights looking for the little boy’s rock, 140 pounds of canine dark matter invariably lies sprawled like a steep speed bump in my path. A couple of weeks ago I heard a professional astronomer say that a colleague of his enjoyed thinking of dark matter as innumerable Louisville Sluggers toppling majestically end over end through space. And unless we are wearing beacons on out heads, we are dark matter too. But people, dogs and Louisville Sluggers can’t account for that 99.99 percent of missing matter in the universe.

A couple of exotic but popular candidates for cosmic dark matter are WIMPs and MACHOs. WIMPs stands for Weakly Interacting Massive Particles. MACHOs are Massive Compact Hallow Objects. The acronyms just go to show that astrophysicists do have a sense of humor.

WIMPs are thought to be extremely small and heavy bits of matter that don’t do much of anything. They just whiz through space, even the space between the atoms in our bodies, without causing any harm or leaving any trace. WIMPs could form a vast sea of material in which all ordinary visible mater exists. Millions of WIMPs could be passing through you as you read this. No one has found any WIMPs yet, but a lot of physicists are looking.

MACHOs are thought to be large lumps of ordinary matter, more like Sancho, but each about the size of Jupiter, all hanging around in the dark spaces on the outer edges of the galaxies (an area called the galactic halo). No one has found MACHOs either, but the search continues.

Among the wonders exotic dark matter is expected to someday explain is how fast the universe is flying apart and how stars and galaxies came into being. But it could also forever change the way we look at the insignificant fraction of "brighter" matter of which we are made.