Dark Matter: A Cosmic Ghost Story [Around the Globe]

There is a ghost in the universe. Many things are “invisible” because they do not emit light at wavelengths we can see: newborn stars glowing in the infrared, microwaves from the start of the universe, and rapidly spinning neutron stars all shine, they just do it in parts of the spectrum that human eyes are blind to. But all of these things still emit light, of a kind. We could call this “type one” invisibility: invisible from the point of view of human beings but perfectly visible if you can “see” with infrared, or microwave, or radio wave eyes. But over the last few decades, astronomers have become aware that there is something truly invisible haunting the spaces of our cosmos. Dark matter is so invisible, we still have not got a glimpse of it—at all—and can only guess at its existence because we see the effects it has on the normal matter that surrounds it. As far as we can tell, dark matter is a kind of “type two” invisible, completely dark across the entire electromagnetic spectrum.