Listen here [6:12m]:
What’s the facts:
Dark matter is a mysterious substance that we cannot see, touch, smell, hear, or taste (although it might not taste so bad). Yet there is about 6 times more of it in the Universe than normal matter, stuff like protons, electons, and neutrons that the stars, planets, and people are made of. If we can’t see it, how do we know dark matter is out there? Well, dark matter still has a gravitational force associated with it, and it is that “extra attraction” that reveals its existence. The motion of galaxies in clusters and the orbits of stars around the centers of galaxies (including our own) both reveal the presence of dark matter. Perhaps more spectacularly, a large amout of dark matter – say, associated with a whole cluster of galaxies – acts as a gravitational lens, focusing and warping light from galaxies behind the cluster like a fun house mirror (for an interesting example, see this picture of Abell 2218). Despite not being able to see dark matter, scientists know quite a bit about its properties – that its probably not just “dim” normal matter (like neutrinos or brown dwarfs); that it is mostly “cold” (not moving close to the speed of light); that it interacts very weakly with regular matter (as illustrated with the Bullet Cluster collision); and that it is distributed over large length scales like the size of a galaxy, as opposed to the size of a planet (sorry, no pulling dark matter tricks on your little brother at home). Because it is makes up so much of the known Universe, there are many experiments now underway to try to detect dark matter through their infrequent collisions with other regular matter particles. Hopefully, within the next few years we may have our first detection of the stuff that makes up most of the mass in the Universe!
Original air date 23 June 2009.