The theory is that dark matter is made up of particles that can't interact with the electromagnetic force, and thus can't be revealed with light. But they do interact gravitationally--in fact, it's the gravitational pull of dark matter that stops galaxies from flying apart as they rotate. Astronomers think there is five times as much dark matter in the universe as there is ordinary matter, even though we can't see it. Since both the dark and the visible forms of matter are affected by gravity, they tend to cluster together.
If we can't see dark matter, how is it detected? By looking at the distorting effect that dark matter has on the light emitted by nearby galaxies.
We'll have more answers soon, since--now that they've seen the Higgs Boson, CERN will now start searching for individual particles of dark matter.
Meanwhile, Curiosity isn't the only space vehicle that's exploring the surface of Mars (NOTE: Subscribers can still listen to this show). NASA's long-lived rover Opportunity, launched in 2003, has returned an image of tiny round rocks, about one-eighth of an inch in diameter, that astronomers have nicknamed "blueberries."
Science Daily quotes NASA's Steve Squyres as saying, "This is one of the most extraordinary pictures from the whole mission. (The 'Kirkwood' area of Mars) is chock full of a dense accumulation of these small spherical objects. Of course, we immediately thought of the blueberries, but this is something different. We never have seen such a dense accumulation of spherules in a rock outcrop on Mars."
NASA's John Callas is quoted as saying, "The rover is in very good health considering its 8 ½ years of hard work on the surface of Mars."
We might as well ask, who are the Visitors? We're proud to say that WE'RE researching that question, by interviewing some of the many people who have interacted with them, just for our wonderful and loyal subscribers.