Everything you've ever seen — every star, planet, person and speck of dust — adds up to only about a sixth of the matter in the universe. The rest is "dark matter," an invisible substance we've never directly detected but whose gravity holds galaxies together. We lay out the evidence that it's really there, what it might be, and why scientists take a ghost so seriously.

You'll come away understanding why galaxies spin in a way that shouldn't be possible without extra unseen mass, how gravitational lensing lets us map the invisible, why dark matter isn't just ordinary stuff that's hard to see, and how science handles a problem it hasn't yet solved.

Talking Science is sponsored by Kinsoft. Visit www.kinsoft.com.au

Further reading: the CERN "Dark matter ...