Existe la materia obscura?

A little over a year ago, Sean Carroll over at Cosmic Variance brought up the idea of Dark Photons, a mysterious analog to dark matter and dark energy that would have created a sort of shadow version of electromagnetism. (The photon is the gauge boson of electromagnetism, meaning that it's the particle that mediates the electromagnetic force. A dark photon would, by analogy, give rise to some similar interaction. See Particle Physics Fundamentals for a brief introduction to this concept ... more to come on this soon, I think.)

Anyway, back to dark photons. The goal of the original paper (written by Carroll and others) was to propose a mechanism by which dark matter and dark energy could be part of a larger, more rich (and interesting) set of physical behaviors in their own right. They weren't the only ones to do this, as various kinds of dark fields and interactions had been proposed by others.

The problem, of course, is that so far we haven't actually observed either dark matter or dark energy directly, but know about them only by their gravitational interactions with other matter. This makes the proposition of any distinct properties to be highly speculative. Carroll himself is very upfront about this, making the analogy that it "invoke[s] the tooth fairy (propose an extremely speculative idea or hope for some possible but unprovable result)." He points out that this there is an unspoken (although he actually spoke it, so I suppose it's not unspoken) rule that this is sometimes allowed, but can only be done once per paper ... so they introduced the idea of dark photons and then hinted at expanding the idea, but never got back to it.

Well, now someone has, by expanding the dark photon idea into an entire range of atomic dark matter. As Carroll explains it, this new proposal has some implications for why it would be so hard to detect dark matter. Atomic dark matter, like ordinary atomic matter, is electrically neutral. (In ordinary matter, the positive charge of the nucleus and the negative charge of the electrons result in a net electrically neutral atom. Similar mechanisms would, presumably, be at work in atomic dark matter.) This means that attempts to detect it mean you don't have to particularly worry about the fact that you haven't observed them electromagnetically ... the only real interaction that you'd expect is gravitational.

This is important, because there are some who claim that Earth-bound experiments have detected dark matter already, but others think that this is premature. (The results are solid; the interpretation is not nearly as clear.) But why would we detect the effect of dark matter particles, but not detect the particles themselves? Because the dark matter doesn't really emit electromagnetic radiation in any form that we normally think of it, perhaps?

It's certainly possible, though a lot of work needs to be done from these early ideas to developing a full theory. One thing to keep in mind, of course, is that the atomic dark matter is only electrically neutral in its most basic form. The atoms follow quantum physical laws that dictate how many electrons they need to be stable. In trying to reach this stability, the atoms interact with other atoms, forming ions and molecules. If dark matter is composed of atomic dark matter, then the question that remains is how complex the structure could get. Are there dark molecules? Why not dark organic molecules? Or dark amino acids? Or dark life? (I believe the Robert J. Sawyer science fiction novel Starplex features just this concept.)

Or if, alternatively, there is a limit to how complex the dark atoms can get, then we have to wonder what limitating features are at work and why they aren't at work in ordinary atomic matter.

This is potentially a rich minefield for scientists and, hopefully, future research will provide some experimental evidence that can help determine what physical meaning, if any, this research has.