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AnonTechie writes that Ars Technica is running a story If Dark Matter is Really Axions, we Could Find Out Soon:

From observations of the Milky Way galaxy, we’ve learned that in any given cubic meter of space, even the particular cubic meter that snugly fits your seated form as you read this article, there’s a small amount of matter—only about 50 proton masses worth—passing through in any given moment. But unlike the particles that make up your seated form, this matter doesn’t interact [electromagnetically]. It doesn’t reflect light, it isn’t repelled by solid objects, it passes right through walls. This mysterious substance is known as dark matter.

Since there’s so little of it in each cubic meter, you would never notice its presence. But over the vast distances of space, there’s a lot of cubic meters, and all that dark matter adds up. It’s only when you zoom out and look at the big picture that dark matter’s gravitational influence becomes apparent. It’s the main source of gravity holding every galaxy together; it binds galaxies to one another in clusters; and it warps space around galaxy clusters, creating a lensing effect.

But despite its importance to the large-scale structure of the Universe, we still don’t know what dark matter really is. Currently, the best candidate is WIMPs, or Weakly Interacting Massive Particles (Which makes sense, now that we know it’s not MAssive Compact Halo Objects, or MACHOs). But WIMPs are not the only option—there are quite a few other possibilities being investigated ( http://arstechnica.com/science/2014/09/exploring-the-monstrous-creatures-at-the-edges-of-the-dark-matter-map/1/ ). Some of them are other kinds of massive particles, which would constitute cold dark matter, while others aren’t particles at all. ( http://arstechnica.com/science/2014/11/looking-for-a-different-sort-of-dark-matter-with-gps-satellites/ )

[Paper]: http://www.pnas.org/content/early/2015/01/07/1308788112.full.pdf+html

[Abstract]: http://www.pnas.org/content/early/2015/01/07/1308788112