Does Earth Live In a Dark Matter-Free Zone?

I wish more people were way excited about dark matter—at least as into it as they were the Higgs boson hunt. My sense, however, is that they aren’t—though I did get a kick out of CNN’s attempt to wrangle interest over the course of an afternoon earlier this year after a possible detection from ISS-based equipment.

It makes sense though. The dark matter hunt is dispersed among projects all over the globe and doesn’t boast that same extreme concentration of billions of dollars of funding. Relative to the many billions spent on hunting the Higgs at the Large Hadron Collider, an individual dark matter detection experiment is couch change. The truth is that science in a way very often decides for us what’s important via money, which sucks.

Dark matter has a great punchline and, in the unveiling of new dark matter information, it’s handy to start with: Dark matter implies that we are the ghosts of the universe. What I mean by “ghosts” is that most of the matter in the universe is dark matter that’s invisible to us; it avoides nearly any sort of interaction with “normal” matter, save for gravity. It passes through our world, or we pass through its world, untouched beyond gravitational tugs. Ghosts. There could be a pot of dark matter pouring through you right now as you’re reading this post (via light particles, which don't interact with dark matter) of our matter and you’d have no reason to know.

But, that lack of interaction doesn’t make dark matter irrelevant. In fact, it shapes galaxies—or even makes galaxies possible. It’s thought that dark matter is the stuff around which matter coalesced to make objects, structure, and even material differentiation possible. Dark matter gives the universe life, or creates the conditions for it. Without dark matter, the universe would look about the same at any given point.

The catch is that we don’t know what dark matter is or even really where it is. There’s a lot of theories about both things, but dark matter being, well, dark, it’s very hard to make observations of it. Everything we have so far is indirect observation based on what dark matter does to normal matter via gravity.

So it’s very crucial to our understanding of what dark matter is (in all sorts of senses, from particle makeup to behavior) to know where it is. A great deal of attention is thus given to its possible distributions within galaxies; if we know dark matter, we also know how we got here, to this current universe that we see.

There are two ideas for that dark matter density distribution: it’s either spread out evenly within the galaxy, or it’s concentrated at the center. That is, as you go from the edge of the galaxy to the center of it, the dark matter density will increase. The disagreement, known as the “core/cusp debate,” is roughly demarcated between astronomers making observations on galaxies with telescopes and theorists running simulations on supercomputers. Yesterday, a team from the University of Texas claimed an answer in a paper in the Astrophysical Journal Letters: dark matter is concentrated at the center. So there.

Since the debate began in the 90s with the advent of high-power modelling, researchers have amassed both new data taken from telescopes and fancier and more impartial computer models that are, “not assuming core or cusp theory is right,” UT grad student John Jardel  says, “but just asking ‘what is it?'” At first, it appeared to be both: Some galaxies had an even distribution, while others had a concentrated core. But after averaging it all out, the general result was that the dark matter density was not constant across galaxies. It dissipates as you move outward.

So, as you can see above, we’re interacting with a lot less dark matter than most of the galaxy, which is likely bad news for the growing assembly of Earthbound dark matter detectors. Though not as bad as a universe without dark matter.

@everydayelk