In the dark

 "Okay, that's cool, but what the hell am I looking at?"

That was my reaction to a press release last week from the Harvard-Smithsonian Center for Astrophysics about a new study of the distribution of dark matter in the universe.  Turns out it's not uniform, which is what I'd have expected given that it apparently doesn't interact with anything except via gravitation (although I hardly need to point out that my opinion on the matter counts for next to nothing because I'm not a physicist).  It exists in filaments and haloes, where the majority of galaxies are concentrated.  Here's one of the images they generated:

I know you can't read much into appearances, but I was immediately struck by how much this image, especially the right-hand part, looks like a neural net.  (I'm just waiting for the woo-woos to latch onto this and claim that this proves the universe is a giant brain.)

"Amongst the things we’ve learned from our simulations is that gravity leads to dark matter particles 'clumping' in overly dense regions of the universe, settling into what’s known as dark matter haloes," said study lead author Sownak Bose.  "These can essentially be thought of as big wells of gravity filled with dark matter particles.  We think that every galaxy in the cosmos is surrounded by an extended distribution of dark matter, which outweighs the luminous material of the galaxy by between a factor of 10-100, depending on the type of galaxy.  Because this dark matter surrounds every galaxy in all directions, we refer to it as a 'halo.'"

So this could be a partial explanation for structures like the Boötes Void, a region of space so empty that (in the words of astronomer Greg Aldering) if the Milky Way was at the center of it, we wouldn't have known about the existence of other galaxies until the 1960s.  It's about 236,000 cubic megaparsecs -- equivalent to a cube 61 trillion parsecs on each side -- and, as of this writing, seems to contain only sixty galaxies.

That, my friends, is a whole lot of nothing.

The distribution of matter in space is clumpy and irregular.  Whether this drives the distribution of dark matter, or it's the other way around (the distribution of dark matter drives the arrangement of ordinary matter in the cosmos) is unknown.

Because that's the trouble, here, to go back to my initial question.  We've got some wonderful pictures of dark matter haloes and filaments, but what the hell is it?  I know the physicists have been working on this question ever since astronomer Vera Rubin demonstrated its existence back in the 1990s, but for cryin' in the sink, it makes up 83% of the mass of the universe, and we still don't have a good idea of what it's made of or how it interacts (again, other than its gravitational signature, which is how it was detected in the first place).

But what dark matter actually is still lies in the realm of speculation.  "Ground-based telescopes like the Very Energetic Radiation Imaging Telescope Array System (VERITAS) can be used for this purpose [detecting dark matter], too." said Jie Wang, who co-authored the study.  "And, pointing telescopes at galaxies other than our own could also help, as this radiation should be produced in all dark matter haloes.  With the knowledge from our simulation, we can evaluate many different tools to detect haloes—gamma-ray, gravitational lensing, dynamics.  These methods are all promising in the work to shed light on the nature of dark matter particles."

So the upshot is there's a network of invisible stuff spreading through the entire universe, perhaps organizing the distribution of ordinary matter, but for sure surrounding and penetrating everything there is.  Without interacting with it in any way other than gravity (as far as we can tell).

Which is a hell of a mystery, isn't it?

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This week's Skeptophilia book-of-the-week is about one of the most terrifying viruses known to man: rabies.

In Rabid: A Cultural History of the World's Most Diabolical Virus, by Bill Wasik and Monica Murphy, we learn about the history and biology of this tiny bit of protein and DNA that has, once you develop symptoms, a nearly 100% mortality rate.  Not only that, but it is unusual amongst pathogens at having extremely low host specificity.  It's transmissible to most mammal species, and there have been cases of humans contracting rabies not from one of the "big five" -- raccoons, foxes, skunks, bats, and dogs -- but from animals like deer.

Rabid goes through not only what medical science has to say about the virus and the disease it causes, but its history, including the possibility that it gave rise to the legends of lycanthropy and werewolves.  It's a fascinating read.

Even though it'll make you a little more wary of wildlife.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]