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. 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. Some of them are other kinds of massive particles, which would constitute cold dark matter, while others aren’t particles at all. Axions, theoretical particles that were originally predicted to solve a tricky problem involving the strong nuclear force, happen to have just the right properties to be a good candidate for dark matter. Leslie Rosenberg, a physicist at the University of Washington, Seattle, recently wrote an overview of the experiments being done to investigate the possibility of axions being dark matter for the journal PNAS. Hot or Cold? Among the various models of dark matter, there are two overarching categories: Hot (HDM) and Cold Dark Matter (CDM). The hot variety gets its name because its particles would be whipping around at incredibly high speeds, up to significant fractions of the speed of light. But hot dark matter seems to be a dead end as a possibility. If particles were traveling that fast, most of them would be able to escape the gravitational pull of their host galaxy. Instead, dark matter forms into nice, spherical halos around every galaxy—which means that it's probably cold. Full article via Ars Technica.